North Korea has periodically asserted its need for a nuclear deterrent since the Korean War, when the United States threatened to use nuclear weapons against it.
The UN Security Council has issued a series of sanctions against North Korea’s nuclear program, including Resolution 1695, Resolution 1718, Resolution 1874, and Resolution 2087. The Six-Party Talks between North Korea, South Korea, Japan, China, Russia, and the United States began in 2003 with the goal of denuclearizing the Korean Peninsula. However, these talks have been suspended since April 2009. Tensions have continued to rise since North Korea torpedoed the South Korean naval ship, the Cheonan, and shelled the island of Yeonpyeong in 2010. North Korea possesses full nuclear fuel cycle capabilities, including a presumed basic capability to enrich uranium, although the functionality of its centrifuges has not yet been substantiated to the outside world. North Korea is building a small light water reactor (LWR) at Yongbyon, which will be fueled by the enriched uranium after it is completed. 
1950s to 1960s: Early Developments
In the early 1950s, North Korea began developing the institutional capability to train personnel for its nuclear program. In December 1952, the government established the Atomic Energy Research Institute and the Academy of Sciences, but nuclear work only began to progress when North Korea established cooperative agreements with the Soviet Union.  Pyongyang signed the founding charter of the Soviet Union’s Joint Institute for Nuclear Research in February 1956, and began to send scientists and technicians to the USSR for training shortly thereafter. In 1959, North Korea and the Soviet Union signed an agreement on the peaceful use of nuclear energy that included a provision for Soviet help to establish a nuclear research complex in Yongbyon, North Pyongan Province. 
In the early 1960s, the Soviet Union provided extensive technical assistance to North Korea in constructing the Yongbyon Nuclear Research Center, which included the installation of a Soviet IRT-2000 nuclear research reactor and associated facilities. North Korea used this small research reactor to produce radioisotopes and to train personnel.  Although the cabinet and the Academy of Sciences were given operational and administrative oversight of the nuclear facilities, then-North Korean leader Kim Il Sung retained ultimate control of the nuclear program and all decisions associated with weapons development.
Although bolstered by early assistance from Moscow, and to some extent Beijing, North Korea’s nuclear program developed largely without significant foreign assistance. Reportedly, Kim Il Sung asked Beijing to share its nuclear weapons technology following China’s first nuclear test in October 1964, but Chinese leader Mao Zedong refused.  In any case, shortly thereafter, North Korean relations with China began to deteriorate.
1970s to 1993: Indigenous Development Under the Radar of the International Community
In the late 1960s, North Korea expanded its educational and research institutions to support a nuclear program for both civilian and military applications. By the early 1970s, North Korean engineers were using indigenous technology to expand the IRT-2000 research reactor, and Pyongyang had begun to acquire plutonium reprocessing technology from the Soviet Union.  In July 1977, North Korea signed a trilateral safeguards agreement with the IAEA and the USSR that brought the IRT-2000 research reactor and a critical assembly in Yongbyon under IAEA safeguards. The Soviets were included in the agreement because they supplied the reactor’s fuel. 
The early 1980s was a period of significant indigenous expansion, when North Korea constructed uranium milling facilities, fuel rod fabrication complex, and a 5MW(e) nuclear reactor, as well as research and development institutions. Simultaneously, North Korea began experimenting with the high explosives tests required for building the triggering mechanism of a nuclear bomb. By the mid-1980s, Pyongyang had begun constructing a 50MW(e) nuclear reactor in Yongbyon, while also expanding its uranium processing facilities. 
Pyongyang also explored the acquisition of light water reactor technology in the early to mid-1980s. This period coincided with the expansion of North Korea’s indigenously designed reactor program, which was based on gas-graphite moderated reactors similar in design to the Calder Hall reactors first built in the United Kingdom in the 1950s. Pyongyang agreed to sign the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) as a non-nuclear weapon state in December 1985 in exchange for Soviet assistance constructing four LWRs. 
In September 1991, U.S. President George H. W. Bush announced that the United States would withdraw its nuclear weapons from South Korea, and on 18 December 1991, President Roh Tae Woo declared that South Korea was free of nuclear weapons.  North Korea and South Korea then signed the Joint Declaration on the Denuclearization of the Korean Peninsula, whereby both sides promised they would «not test, manufacture, produce, receive, possess, store, deploy or use nuclear weapons.» The agreement additionally bound the two sides to forgo the possession of «nuclear reprocessing and uranium enrichment facilities.» The agreement also provided for a bilateral inspections regime, but the two sides failed to agree on its implementation. 
The 1994 Crisis and the Agreed Framework
North Korea finally signed an IAEA safeguards agreement on 30 January 1992, and the Supreme People’s Assembly ratified the agreement on 9 April 1992. Under the terms of the agreement, North Korea provided an «initial declaration» of its nuclear facilities and materials, and provided access for IAEA inspectors to verify the completeness and correctness of its initial declaration.  Six rounds of inspections began in May 1992 and concluded in February 1993. Pyongyang’s initial declaration included a small plutonium sample (less than 100 grams), which North Korean officials said was reprocessed from damaged spent fuel rods that were removed from the 5MW(e) reactor in Yongbyon-kun. However, IAEA analysis indicated that Korean technicians had reprocessed plutonium on three occasions—in 1989, 1990, and 1991.  When the Agency requested access to two suspect nuclear waste sites, North Korea declared them to be military sites and therefore off-limits. 
After the IAEA was denied access to North Korea’s suspect waste sites in early 1993, the Agency asked the United Nations Security Council (UNSC) to authorize special ad hoc inspections. In reaction, North Korea announced its intention to withdraw from the NPT on 12 March 1993.  Under the terms of the treaty, a state’s withdrawal does not take effect until 90 days after it has given notice. Following intense bilateral negotiations with the United States, North Korea announced it was suspending its withdrawal from the NPT one day before the withdrawal was to take effect. Pyongyang agreed to suspend its withdrawal while talks continued with Washington, but claimed to have a special status in regard to its nuclear safeguards commitments. Under this special status, North Korea agreed to allow the continuity of safeguards on its present activities, but refused to allow inspections that could verify past nuclear activities. 
As talks with the United States over North Korea’s return to the NPT dragged on, North Korea continued to operate its 5MW(e) reactor in Yongbyon. On 14 May 1994, Korean technicians began removing the reactor’s spent fuel rods without the supervision of IAEA inspectors.  This action worsened the emerging crisis because the random placement of the spent fuel rods in a temporary storage pond compromised the IAEA’s capacity to reconstruct the operational history of the reactor, which could have been used in efforts to account for the discrepancies in Pyongyang’s reported plutonium reprocessing.  U.S. President Bill Clinton’s administration announced that it would ask the UNSC to impose economic sanctions; Pyongyang responded that it would consider economic sanctions «an act of war.» 
The crisis was defused in June 1994 when former U.S. President Jimmy Carter traveled to Pyongyang to meet with Kim Il Sung. Carter announced from Pyongyang that Kim had accepted the broad outline of a deal that was later finalized as the Agreed Framework in October 1994.  Under the agreement, North Korea agreed to freeze work at its gas-graphite moderated reactors and related facilities, and to allow the IAEA to monitor that freeze. Pyongyang was also required to «consistently take steps to implement the North-South Joint Declaration on the Denuclearization of the Korean Peninsula,» and to remain a party to the NPT. In exchange, the United States agreed to lead an international consortium to construct two light water power reactors, and to provide 500,000 tons of heavy fuel oil per year until the first reactor came online with a target date of 2003. Furthermore, the United States was to provide «formal assurances against the threat or use of nuclear weapons by the U.S.» 
2001 to 2003: Collapse of the Agreed Framework and Withdrawal from the NPT
While the Agreed Framework froze North Korea’s plutonium program for almost a decade, neither party was completely satisfied with either the compromise reached or its implementation. The United States was dissatisfied with the postponement of safeguards inspections to verify Pyongyang’s past activities, and North Korea was dissatisfied with the delayed construction of the light water power reactors.
After coming to office in 2001, the George W. Bush administration initiated a North Korean policy review, which it completed in early June. The review concluded that the United States should seek «improved implementation of the Agreed Framework, verifiable constraints on North Korea’s missile program, a ban on missile exports, and a less threatening North Korean conventional military posture.»  From Washington’s perspective, «improved implementation of the Agreed Framework» meant an acceleration of safeguards inspections, even though the agreement did not require Pyongyang to submit to full safeguards inspections to verify its past activities until a significant portion of the reactor construction was completed, but before the delivery of critical reactor components.
The international community also became concerned that North Korea might have an illicit highly enriched uranium (HEU) program. In the summer of 2002, U.S. intelligence reportedly discovered evidence of transfers of HEU technology and/or materials from Pakistan to North Korea in exchange for ballistic missiles technology.  (Later, in early 2004, it was revealed that Pakistani nuclear scientist Dr. A. Q. Khan had sold gas-centrifuge technology to North Korea, Libya andIran.) 
In October 2002, bilateral talks between the United States and North Korea finally resumed when U.S. Assistant Secretary of State for East Asia and Pacific Affairs James Kelly visited Pyongyang.  During the visit, Kelly informed First Vice Foreign Minister Kang Sok Chu and Vice Foreign Minister Kim Kye Kwan that Washington was aware of a secret North Korean program to produce HEU. The U.S. State Department claimed that North Korean officials admitted to having such a program during a second day of meetings with Kelly, but North Korea later argued that it had only admitted to having a «plan to produce nuclear weapons,» which Pyongyang claimed was part of its right to self-defense. 
The United States responded in December 2002 by suspending heavy oil shipments, and North Korea retaliated by lifting the freeze on its nuclear facilities, expelling IAEA inspectors monitoring that freeze, and announcing its withdrawal from the NPT on 10 January 2003.  Initially, North Korea claimed it had no intention of producing nuclear weapons, and that the lifting of the nuclear freeze was necessary to generating needed electricity.
2003 to 2006: New Crises, and the Beginning and End of the Six-Party Process
In early 2003, U.S. intelligence detected activities around the Radiochemisty Laboratory, a reprocessing facility in Yongbyon, which indicated that North Korea was probably reprocessing the 8,000 spent fuel rods that had been in a temporary storage pond.  In September 2003, a North Korean Foreign Ministry spokesman said that North Korea had completed the reprocessing of this spent fuel—this would have given North Korea enough plutonium for approximately four to six nuclear devices.  In January 2004, a delegation of invited U.S. experts confirmed that the canisters in the temporary storage pond were empty. 
In April 2003, a multilateral dialogue began in Beijing with the aim of ending Pyongyang’s nuclear weapons program. Initially trilateral in format (China, North Korea and the United States), the process expanded to a six-party format with the inclusion of Japan, Russia and South Korea. The first round began in August 2003. Six months later, in February 2004, the second round of talks was held, and a third round followed in June 2004. However, tensions between the parties—particularly the United States and North Korea—caused the talks to stall for more than a year, restarting in July 2005.
While the six-party process stagnated, North Korea shut down its 5MW(e) reactor in April 2005 and removed the spent fuel.  The reactor had been operating since February 2003, meaning that it could have produced enough plutonium for between one and three nuclear devices from its spent fuel. However, it would take a few months for North Korean engineers to extract the plutonium from the spent fuel rods. In July 2005, satellite imagery indicated that the reactor had begun operations once again. 
On 19 September 2005, the fourth round of Six-Party Talks concluded and the six parties signed aStatement of Principles, whereby North Korea would abandon its nuclear programs and return to the NPT and the IAEA safeguards regime at «an early date.» The United States stated that it had no intention of attacking North Korea with nuclear or conventional weapons, and Washington affirmed that it has no nuclear weapons deployed in South Korea. The parties also agreed that the 1992 Joint Declaration on the Denuclearization of the Korean Peninsula, which prohibited uranium enrichment or plutonium reprocessing, should be observed and implemented. 
Although hailed as a breakthrough by some participants, the viability of the Statement of Principles was immediately brought into question by North Korean and U.S. actions. The parties disagreed over the implications of the Statement of Principles for LWR transfer to North Korea. While Pyongyang argued that the six-party statement permitted LWR transfer, Washington countered that this was not guaranteed under the statement and could only occur after North Korea had dismantled its existing nuclear program. Shortly after signing the agreement in Beijing, the U.S. Treasury Department announced that U.S. financial institutions were barred from having correspondent accounts with Banco Delta Asia (BDA), a Macao-based bank, which it accused of assisting North Korea in illicit transactions.  North Korea asserted that unless the so-called “sanctions” were lifted, Pyongyang would not carry out its part of the September 2005 agreement.  Due to these and other disagreements, the Six-Party Talks stalemated, and the Statement of Principles remained dormant for more than 18 months.
2006 to 2011: A Nuclear Test, Failed Negotiations, and Another Nuclear Test
The nuclear crisis on the Korean Peninsula continued to deteriorate throughout 2006, reaching a low point in October when North Korea conducted its first nuclear test at 10:35AM (local time) at Mount Mantap, Punggye-ri, Gilju-gun, North Hamgyeong Province.  The Korean Central News Agency (KCNA) announced that the test was conducted at a «stirring time when all the people of the country are making a great leap forward in the building of a great prosperous powerful socialist nation.»  The North Korean nuclear test did not, however, produce a significant yield. The yield from this test appeared to be less than 1 kiloton. North Korea was reportedly expecting at least a 4 kiloton yield, possibly indicating that the North Korean plutonium program still had a number of technical hurdles to overcome before it would have a nuclear warhead. 
Immediately following the test, UNSC Resolution 1718 imposed sanctions on North Korea.  After intense diplomatic activities by the Chinese government and others involved in the Six-Party process, the parties met again in December 2006 following a hiatus of more than a year. However, these talks ended without any sign of progress.  In what appeared to be a breakthrough in the negotiations, the six parties in February 2007 agreed on the Initial Actions for the Implementation of the Joint Statement, whereby North Korea agreed to abandon all of its nuclear weapons and existing nuclear programs, and to return to the NPT and the IAEA safeguards regime, in exchange for a package of incentives that included the provision of energy assistance to North Korea by the other parties.  The agreement also established a 60-day deadline during which North Korea was to shut down and seal its main nuclear facilities at Yongbyon-kun under IAEA supervision. Additionally, the United States agreed to release the approximately $25 million in North Korean assets held at the Macao-based Banco Delta Asia.  However, the BDA part of the agreement again became a sticking point; much of the international financial community, concerned about the possible legal ramifications of dealing with a bank that was technically still under U.S. sanctions, refused to take part in the transfer of the funds. The issue was eventually resolved when a Russian bank agreed to transfer the funds in June 2007. 
After the February 2007 agreement, North Korea extended invitations to IAEA officials, opening the door to reestablishing its relationship with the Agency. In July 2007, North Korea began shutting down and sealing it main nuclear facilities at Yongbyon-kun under IAEA supervision.  Further progress was made in the Six-Party Talks when the parties adopted the Second Action Plan, calling on North Korea to disable its main nuclear facilities and submit a complete and correct declaration of all its nuclear programs by 31 December 2007.  While disablement activities on North Korea’s three key plutonium production facilities at Yongbyon-kun (the 5MW(e) experimental reactor, the Radiochemistry Laboratory and the Fuel Fabrication Plant) progressed, North Korea failed to meet the 31 December deadline to submit its declaration.  Sharp disagreements over North Korea’s past illicit procurement efforts and controversies surrounding suspected North Korean nuclear cooperation with Syria proved to be the key sticking points.
Almost six months past the deadline, on 26 June 2008, North Korea submitted its much-awaited declaration.  While the contents of North Korea’s declaration have not been disclosed to the public, various media reports claimed that the declaration failed to address both North Korea’s alleged uranium enrichment program and suspicions of its nuclear cooperation with countries such as Syria.  Despite problems with the declarations, the Bush administration notified the U.S. Congress that it planned to remove North Korea from the U.S. list of state sponsors of terrorism, and also issued a proclamation lifting some sanctions under the Trading with the Enemy Act.  Following the U.S. government’s actions, North Korea demolished the cooling tower at the Yongbyon 5MW(e) reactor, an event broadcasted by international media. 
Delays with the U.S. removal of North Korea from the state sponsors of terrorism list contributed to North Korean delays in meeting its own commitments, and eventually Pyongyang announced in late August 2008 that it had restored the nuclear facilities in Yongbyon-kun, and barred international inspectors from accessing the site.  On 11 October 2008, the United States finally dropped North Korea from the terrorism list after reaching a deal in which North Korea agreed to resume the disabling of its nuclear facilities, and to allow inspectors access to the nuclear sites.  The six parties then resumed negotiations to map out a verification plan in Beijing in December 2008. These negotiations focused on ways to verify the disablement of North Korea’s nuclear program, including taking nuclear samples. However, the negotiations failed to reach an agreement on a verification protocol, and the issue remains stalled. 
After a dispute over rocket launches in March 2009, North Korea kicked out IAEA and U.S. inspectors and began to rebuild the Yongbyon 5MW(e) reactor for the purpose of reprocessing plutonium from its spent fuel rods, in contravention of its previous promises at the Six-Party Talks. On 25 May 2009, North Korea conducted its second nuclear test. KCNA announced that Pyongyang had carried out the nuclear test, and that it «was safely conducted on a new higher level in terms of its explosive power and technology of its control.»  Initial estimates from the U.S. government showed the test causing seismic activity equivalent to a magnitude of 4.7 on the Richter Scale, and located close to the site of the first nuclear test in 2006.  Early estimates pointed to a possible yield for the test of between 2 and 8 kilotons, with about 4 kilotons being most likely.  The United Nations Security Council released Resolution 1874; in response Pyongyang announced that «the processing of uranium enrichment will be commenced.» North Korea further indicated that it did not intend to return to the Six-Party Talks, and asserted that it would not be bound by agreements made earlier through this forum. 
Tensions continued to rise in 2010 and 2011. North Korean leader Kim Jong Il visited China three times within one year, each time indicating he was willing to proceed with denuclearization efforts; however North Korea also engaged in several military confrontations with the South.  In March 2010, North Korea torpedoed a South Korean ship killing 46 sailors, and in November of the same year it shelled Yeonpyeong Island, killing four South Koreans, including two civilians.  On 15 March 2011, Pyongyang announced its willingness to return to the Six-Party Talks without preconditions, and agreed to discuss its uranium enrichment program. 
However, in March 2010, North Korea announced the construction of a light-water reactor (LWR) at Yongbyon.  U.S. nuclear expert Siegfried Hecker confirmed that construction for a 25-30MW(e) experimental LWR had commenced during his November 2010 visit. In November 2011 analysts estimated that the experimental LWR may be externally complete within the next year, but operations are unlikely to begin for another two to three years as machinery and equipment must be loaded and installed.  Additionally, Hecker reported that North Korea had completed the construction of a uranium enrichment facility at Yongbyon with 2,000 P-2 centrifuges in six cascades.  Although satellite imagery showed that activity dbeen halted since late April 2014, new imagery from September 2015 show increased activity, likely indicating increased uranium production. 
Recent Developments and Current Status
The death of Kim Jong Il in December 2011 left much of the world speculating about its impact on North Korea’s nuclear program and the Six-Party Talks. After a series of bilateral talks with the U.S., North Korea announced a moratorium on nuclear testing, uranium enrichment, and long-range missile tests on 29 February 2012 in exchange for food aid.  However, the U.S. withdrew its offer of food aid, after North Korea attempted to launch a satellite into orbit using an Unha rocket on 12 April 2012. The U.S. considers the space launch a violation of the agreement as well as UN Security Council Resolutions 1718 and 1874, because the rocket is not materially different from a long-range ballistic missile.  North Korea launched an additional Unha rocket from the Sohae Center in December 2012. This rocket successfully placed a satellite into orbit, and the UN Security Council followed up with Resolution 2087 demanding North Korea end its nuclear and missile programs.
On 12 February 2013, North Korea conducted a third nuclear test at the Punggye-ri Nuclear Test Facility.  The USGS reported a 5.1 magnitude seismic shock in the vicinity of the test site.  North Korea claimed to have successfully tested a «lighter, miniaturized atomic bomb.» 
In April 2013, North Korean state media announced that Pyongyang was restarting its 5MW graphite-moderated reactor and uranium enrichment plant at Yongbyon.  Though the original cooling tower is now destroyed, satellite analysis confirmed activity consistent with connecting cooling pipes from the 5MW reactor to the adjacent river.  By August 2013, satellite imagery confirmed steam venting from the reactor’s turbine and generator building.  Meanwhile, the external work on the adjacent experimental light water reactor appears to have concluded based on January 2014 satellite imagery; however, it is unlikely the reactor will be fully operational within the next 1-2 years. 
In December 2015, according to state-run Rodong Sinmun, Kim Jong Un claimed thermonuclear capabilities during his visit to the Pyongchon Revolutionary Site. This claim was met with much skepticism from the international community. 
 “North Korea Makes Significant Progress in Building New Experimental Light Water Reactor (ELWR),” 38 North, 14 November 2011, http://38north.org.
 북한개요 2009 [North Korea Introduction 2009] (Seoul: Korea Institute for National Unification, 2009), p.322.
 Park Yong Chae, «관심쏠린 원전기술 / 北발전설비 상당부분 국산화 [Focus on Nuclear Technology/ The North Produces Most Electricity Themselves],» Kyunghyang Shinmun, 21 June 1994, p. 5.
 Gregory Karouv, «A Technical History of Soviet-North Korean Nuclear Relations,» in James Clay Moltz and Alexandre Y. Mansourov, eds., The North Korean Nuclear Program: Security, Strategy, and New Perspectives from Russia (New York: Routledge, 2000), p. 17.
 Joseph S. Bermudez, Jr., «North Korea’s Nuclear Programme,» Jane’s Intelligence Review, Vol. 3, No. 9, September 1991; Joseph S. Bermudez, Jr., «Exposing North Korea’s Secret Nuclear Infrastructure—Part One,» Jane’s Intelligence Review, July 1999.
 Kim Byeong-gu et al., «북핵기술총서-I [North Korean Nuclear Issues and the LWR Project Vol.1],» KAERI/AR-552-99, Technology Center for Nuclear Control, November 1999, http://www.tcnc.kaeri.re.kr.
 Joseph S. Bermudez, Jr., «North Korea’s Nuclear Programme,» Jane’s Intelligence Review, September 1991, p. 406; Joseph S. Bermudez, Jr., «North Korea’s Nuclear Infrastructure,» Jane’s Intelligence Review, February 1994, p. 77.
 Joseph S. Bermudez, Jr., «North Korea’s Nuclear Infrastructure,» Jane’s Intelligence Review, February 1994, p. 79; Jang Jun-ik, «북한핵 미사일 전쟁 [North Korea Nuclear Missile War],» (Seoul: Seomundang, May 1999), pp. 117 and 124; Lee Jong-hun, «핵 재처리시설 이종훈이 불붙이고 장영식이 물 끼얹다 [Nuclear Reprocessing Facility, Accelerated by Lee Jong-hun, and Stopped by Jang Yong-sik],» Shindonga, September 2002, http://www.donga.com.
 Joseph S. Bermudez, Jr., «North Korea’s Nuclear Programme,» Jane’s Intelligence Review, 1 September 1991, p. 409; Joseph S. Bermudez, Jr., «North Korea’s Nuclear Infrastructure,» Jane’s Intelligence Review, February 1994, p. 74; Don Oberdorfer, The Two Koreas: A Contemporary History (Reading, MA: Addison-Wesley, 1997), p. 254.
 Don Oberdorfer, «U.S. Decides to Withdraw A-Weapons From S. Korea; North Korea to Be Pressed to Halt Program,» Washington Post, 19 October 1991, pp. A1 and A19.
 «Joint Declaration of South and North Korea on the Denuclearization of the Korean Peninsula,» Inventory of International Nonproliferation Organizations and Regimes, James Martin Center for Nonproliferation Studies, http://www.nti.org.
 «Joint Declaration of South and North Korea on the Denuclearization of the Korean Peninsula,» Inventory of International Nonproliferation Organizations and Regimes, James Martin Center for Nonproliferation Studies, http://www.nti.org.
 Mark Hibbs, «Isotopics Show Three North Korean Reprocessing Campaigns Since 1975,»Nuclear Fuel, 1 March 1993, pp. 8-9.
 Don Oberdorfer, The Two Koreas: A Contemporary History (Reading, MA: Addison-Wesley, 1997), p. 279; «March Letter and Memorandum from the UN Permanent Representative of the DPRK to the President of the UN Security Council,» United Nations Security Council Document, 17 March 1993, pp. 1-10.
 Don Oberdorfer, The Two Koreas: A Contemporary History (Reading, MA: Addison-Wesley, 1997), p. 280; Gamini Seneviratne, «IAEA Struggling to Stand Firm and Find Face-Saver for North Korea,» Nucleonics Week, 18 March 1993, p. 10; «Letter from Kim Yong-nam, DPRK Minister of Foreign Affairs,» 12 March 1993; David E. Sanger, «West Knew of North Korea Nuclear Development,» The New York Times, 13 March 1993, p. 3.
 R. Jeffrey Smith, «N. Korea Won’t Quit Nuclear Ban Treaty; Inspection of Two Key Sites Still Rejected,» Washington Post, 12 June 1993, p. A1; Don Oberdorfer, Two Koreas: A Contemporary History (Reading, Massachusetts: Addison-Wesley Reading, 1997), pp. 285-286; J. T. Nguyen, «North Korea Postpones Decision to Abandon Nuclear Treaty,» United Press International, 11 June 1993; John Wright, «North Korea Remains in Nuclear Non-Proliferation Treaty,» Associated Press, 12 June 1993.
 Eric Schmitt, «U.S. Delays Taking Steps Over A-Plant,» The New York Times, 16 May 1994, pp. A1 and A3; David E. Sanger, «North Koreans Say Nuclear Fuel Rods Are Being Removed,»The New York Times, 15 May 1994.
 David E. Sanger, «North Korea Foils Efforts to Halt Its Nuclear Plans,» The New York Times, 29 May 1994; Paul Lewis, «UN Told North Korea’s Nuclear Record Can’t Be Retrieved,» The New York Times, 4 June 1994, p. A3.
 Terence Hunt, «U.S. to Seek UN Sanctions against North Korea,» Associated Press, 2 June 1994; Michael R. Gordon, «White House Asks Global Sanctions on North Koreans,» The New York Times, 3 June 1994; Korean Central Broadcasting Agency (Pyongyang), 3 June 1994, in «Vice Foreign Minister’s Statement on U.S.-Orchestrated Pressure on Nuclear Issue,» BBC Summary of World Broadcasts, 6 June 1994, in Lexis-Nexis, http://web.lexis-nexis.com.
 «Kim Promises Not to Expel IAEA Inspectors,» Agence France-Presse, 16 June 1994; David E. Sanger, «Carter Optimistic after North Korea Talks,» The New York Times, p. A10; Kate Webb, «Carter Goes into Second And Crucial Round of Talks with Kim Il Sung,» Agence France-Presse, 17 June 1994.
 «Agreed Framework between the United States of America and the Democratic People’s Republic of Korea,» 21 October 1994, via http://www.kedo.org; David E. Sanger, «Clinton Approves a Plan to Give Aid to North Koreans,» The New York Times, 19 October 1994, p. A1; «Secret Annex to U.S.-North Korea Agreement,» Deutsche Presse-Agentur, 20 October 1994; Michael R. Gordon, «U.S.-North Korea Accord Has a 10-Year Timetable,» The New York Times, 21 October 1994, p. A8; Alan Riding, «U.S. and North Korea Sign Pact to End Nuclear Dispute,» The New York Times, 22 October 1994, p. A5; «N. Korea’s Pledge on Graphite-Moderated Reactors Cited,» Japan Economic Newswire, 14 January 1999; Takashi Uemura, «N. Korea’s Concession Revealed,»Asahi News Service, 10 May 1999.
 Alex Wagner, «Bush Outlines Resuming Talks with North Korea,» Arms Control Today, July/August 2001, pp. 23 and 25; Michael Knapik, «U.S. to Pursue Better Execution of U.S.-North Korea Agreed Framework,» Nucleonics Week, Vol. 42, No 24, 14 June 2001.
 Stephen Fidler and Edward Luce, «U.S. Fears North Korea Could Gain Nuclear Capability through Pakistan,» Financial Times, 1 June 2001, p. 1; «Pakistan Denies Export of Nuclear Technology,» Japan Economic Newswire, 5 June 2001.
 «‘Father of Pakistan’s Nuclear Bomb Removed,» Deutsche Presse Agentur, 31 January 2004; «Founder of Pakistan’s Nuke Program Sacked as Advisor,» Kyodo News Service, 31 January 2004; John Lancaster and Kamran Khan, «Pakistanis Say Nuclear Scientists Aided Iran,» The Washington Post, 24 January 2004.
 «U.S. Special Envoy Leaves Pyongyang,» Xinhua News Agency, 5 October 2002.
 Press Statement, Richard Boucher, Spokesman, «North Korean Nuclear Program,» U.S. Department of State, 16 October 2002, http://www.state.gov; James A. Kelly, «U.S.-East Asia Policy: Three Aspects,» Remarks at the Woodrow Wilson Center, Washington, DC, 11 December 2002, http://www.state.gov; «Spokesman for DPRK FM on DPRK Visit of Special Envoy of U.S. President,» KCNA, 7 October 2002, http://www.kcna.co.jp; «미국 대통령 특사는 심히 압력적이고 오만하게 나왔다. / 조선외무성 대변인 [U.S. Presidential Envoy was Arrogant and Pressured Us. / Representatives of DPRK’s Ministry of Foreign Affair],» KCNA, 7 October 2002, http://www.kcna.co.jp.
 Carol Giacomo, «U.S. Says N. Korean Fuel Oil Deliveries Should End,» Reuters, 13 November 2002, http://www.reuters.com; P.S. Suryanarayana, «IAEA Inspectors Leave N. Korea,» The Hindu, 1 January 2003; «Statement of DPRK Government on Its Withdrawal from NPT,» KCNA, 10 January 2003, http://www.kcna.co.jp.
 «Crisis on the Korean Peninsula: Implications for U.S. Policy in Northeast Asia,» Speech by William Perry, Brookings Institution, 24 January 2003.
 David E. Sanger, «North Korea Says It Has Made Fuel from Atom Bombs,” The New York Times, 15 July 2003, http://www.nytimes.com; Jong-Heon Lee, «Analysis: N. Korea’s Nuke Game Going Further,» United Press International, 2 October 2003.
 Senate Committee on Foreign Relations Hearing on «Visit to the Yongbyon Nuclear Scientific Research Center in North Korea,» Siegfried S. Hecker, Senior Fellow, Los Alamos National Laboratory, 21 January 2004, Committee Hearings 108th Congress Second Session — 2004, U.S. Senate Committee on Foreign Relations, http://foreign.senate.gov; Barbara Slavin, «Scientist Describes N. Korea Nuclear Evidence,» USA Today, 22 January 2004.
 «N.K. Plans to Unload Fuel Rods from Reactor This Month,» Yonhap News Agency, 16 April 2005; «N. Korea Moves to Bolster Nuclear Arsenal,» Korea Times, 18 April 2005.
 «Report: North Korea Restarted Nuclear Reactor Before International Nuclear Talks,»Associated Press Worldstream, 21 August 2005.
 «North Korea Agrees to Abandon Nuclear Weapons Program,» U.S. Fed News, 19 September 2005, in LexisNexis, http://www.lexisnexis.com.
 U.S. Department of the Treasury, “Treasury Finalizes Rule against Banco Delta Asia BDA Cut off from U.S. Financial System,” 14 March 2007, http://www.treasury.gov.
 «Nuclear Talks Suspended Indefinitely: Korea,» Agence France-Presse, 11 December 2005.
 «Magnitude 4.3 — North Korea,» USGS, 9 October 2006, http://earthquake.usgs.gov.
 «DPRK Successfully Conducts Underground Nuclear Test,» KCNA, 10 October 2006, http://www.kcna.co.jp.
 Office of the Director of National Intelligence, «Statement by the Office of the Director of National Intelligence on the North Korea Nuclear Test,» 11 October 2009, http://www.dni.gov; Siegfried S. Hecker, «Denuclearizing North Korea,» Bulletin of the Atomic Scientists, May/June 2008, http://iisdb.stanford.edu; Siegfried S. Hecker, «Report on North Korean Nuclear Program,» Center for International Security and Cooperation, Stanford University, 15 November 2006.
 Warren Hoge, «Security Council Supports Sanctions on North Korea,» The New York Times, 15 October 2006, http://www.nytimes.com; Philip Sherwell, «UN Vote Slaps Sanctions on North Korea,»Sunday Telegraph, 15 October 2006.
 «Nuclear Talks with N. Korea End in Failure; Six-Party Process Thrown into Doubt,»Washington Post, 23 December 2006, http://www.washingtonpost.com.
 Audra Ang, «North Korea Agrees to Nuclear Disarmament,» Associated Press, 13 February 2005.
 Audra Ang, «North Korea Agrees to Nuclear Disarmament,» Associated Press, 13 February 2005; Christopher Bodeen, «U.S., North Korea Resolve Macau Bank Dispute as Six-Party Talks Begin,» Associated Press, 19 March 2007.
 «Foreign Ministry Spokesman on Solution to Issue of Frozen Funds,» KCNA, 25 June 2007, http://www.kcna.co.jp; «North Korea Says its Banking Row with Washington Resolved,» Associated Press, 25 June 2007.
 Jae-Soon Chang, «UN Inspectors Verify All N. Korea Nuclear Facilities Closed Beyond Reactor,» Associated Press, 18 July 2007.
 «Deadline For Disabling N.K. Nukes Set At Year-End,» Korea Herald, 4 October 2007.
 Choe Sang Hun and Steven Lee Myers, «North Korea Says It Met Nuclear Disclosure Deadline in Previous Declaration,» The New York Times, 5 January 2008; Blaine Harden, «All Nuclear Efforts Disclosed, N. Korea Says; U.S. Calls Pyongyang’s Declaration Incomplete but Says Negotiations Will Continue,» The Washington Post, 5 January 2008; Paul Richter, «N. Korea Says It Has Met Nuclear Criteria; U.S. Officials Say a Full List of Activities Has Not Been Produced,» Los Angeles Times, 5 January 2008.
 «USA Hails North Korea Nuclear List – Yonhap,» in BBC Monitoring, 26 June 2008.
 Glenn Kessler, «U.S. Ready to Ease Sanctions on N. Korea; Pyongyang Would Have to Acknowledge Evidence About Nuclear Activities,» Washington Post, 11 April 2008.
 Norimitsu Onishi and Edward Wong, «U.S. to Remove North Koreans from Terror List; Nuclear Declaration is Rewarded as Disarmament Effort Advances,» International Herald Tribune, 27 June 2008.
 «N. Korea Destroys Reactor Cooling Tower,» Korea Times, 27 June 2008.
 Choe Sang-Hun, «North Korea Says It Stopped Disabling Nuclear Complex,» The New York Times, 27 August 2008.
 Glenn Kessler, «U.S. Drops North Korea from Terrorism List,» The Washington Post, 12 October 2008.
 Jin Dae-Woong, «Nuke Envoys Fail to Narrow Gaps over Verification Pact,» The Korea Herald, 9 December 2008.
 «KCNA Report on One More Successful Underground Nuclear Test,» KCNA, 25 May 2009, http://www.kcna.co.jp.
 «Magnitude 4.7 — North Korea,» USGS, 25 May 2009, http://earthquake.usgs.gov.
 «Next Phase in the Analysis of the Announced DPRK Nuclear Test,» CTBTO, 27 May 2009, http://www.ctbto.org.
 «North Korea To Push Ahead With Uranium Enrichment,» Asia Pulse, 15 June 2009.
 «Kim Vows to Work on Return to N. Korea Nuclear Talks: Xinhua,» Agence France-Presse, 7 May 2010; «DPRK Top Leader Kim Jong-il Hopes for Early Resumption of Six-Party Talks,»Xinhua, 30 August 2010, http://news.xinhuanet.com; Haksoon Paik, “Kim Jong Il’s Visit to China: Implications for East Asia and the United States,” 38 North, 5 June 2011, http://38north.org.
 Victor Cha, «The Sinking of the Cheonan,» Center for Strategic and International Studies, 22 April 2010, http://csis.org; Hyung-jin Kim and Kwang-tae Kim, «North Korean Shelling Killed 4, Gutted Homes,» Associated Press, 24 November 2010; Seo Yoonjung and Keith B. Richburg, “Two Civilians Killed in North Korean Artillery Attack” Washington Post, 24 November 2010, http://www.washingtonpost.com.
 «North Korea ‘Ready to Discuss Nuclear Enrichment’,» BBC, 15 March 2011, http://www.bbc.co.uk.
 «KCNA on Despicable Inside Story about Megaphone War,» KCNA, 29 March 2010, http://www.kcna.co.jp; «N. Korea to Build Light Water Reactor Soon: State Media,» Agence France-Presse, 29 March 2010.
 “North Korea Makes Significant Progress in Building New Experimental Light Water Reactor (ELWR),” 38 North, 14 November 2011, http://www.38north.org.
 Siegfried S. Hecker, «A Return Trip to North Korea’s Yongbyon Nuclear Complex,» Center for International Security and Cooperation, Stanford University, 20 November 2010, http://iis-db.stanford.edu.
 «Light Water Reactor Construction Progressing at Yongbyon Nuclear Site,» ISIS Report, 5 March 2012, http://isis-online.org; Jack Liu, «North Korea’s Punggye-ri Nuclear Test Site: All Quiet for the Moment,» 38 North, 11 August 2014, http://38north.org; http://38north.org.; Jeffrey Lewis, “Recent Imagery Suggests Increased Uranium Production in North Korea,” 38 North, 12 August 2015, http://www.38north.org.
 Steven Lee Myers and Choe Sang-hun, “North Koreans Agree to Freeze Nuclear Work; U.S. to Give Aid,” New York Times, 29 February 2012, http://www.nytimes.com.
 Evan Ramstad and Laura Meckler, «North Korean Launch Fails,» The Wall Street Journal, 13 April 2012, http://online.wsj.com.
 David E. Sanger and Choe Sang-hun, «North Korea Confirms It Conducted 3rd Nuclear Test,»New York Times, 12 February 2013, http://www.nytimes.com.
 «M5.1 – 23km ENE of Sungjibaegam, North Korea,» U.S. Geological Survey, 12 February 2013, http://earthquake.usgs.gov.
 «제3차 지하핵시험을 성공적으로 진행 [Third Underground Nuclear Test Conducted Successfully],» KCNA, 12 February 2013, http://www.kcna.kp.
 “DPRK to Adjust Uses of Existing Nuclear Facilities,” KCNA, 2 April 2013, http://www.kcna.co.jp.
 Nick Hansen and Jeffrey Lewis, “Satellite Images Show New Construction at North Korea’s Plutonium Production Reactor; Rapid Restart?» 38 North, 3 April 2013, http://38north.org; David Albright and Serena Kelleher-Vergantini, “Monitoring Activity at Yongbyon Nuclear Site,” ISIS Reports, 23 April 2014, http://isis-online.org.
 Nick Hansen and Jeffrey Lewis, “Update on Yongbyon,” 38 North, 11 September 2013, http://38north.org.
 Niko Milonopoulos & Edward D. Blandford, “’Safety First – Not One Accident can Occur:’ Nuclear Safety and North Korea’s Quest to Build a Light Water Reactor,” 38 North, 3 April 2014, http://38north.org.
 Nick Hansen, «Nuclear Safety Problems at North Korea’s Yongbyon Nuclear Facility?» 38 North, 7 April 2014, http://38north.org.
 Kelsey Davenport, «N. Korea Warns of New Nuclear Test,» Arms Control Association, May 2014, http://www.armscontrol.org.
 Jeffrey Lewis, «North Korean Nukes 2.0?» 38 North, 4 April 2014, http://38north.org.
 Jack Liu and Joseph S. Bermudez Jr., “New Activity at North Korea’s Punggye-ri Nuclear Test Site,” 38 North, 24 September 2014, http://38north.org.
 William Mugford and Jack Liu, “North Korea’s Yongbyon Nuclear Facility: New Activity at the Plutonium Production Complex,” 38 North, 8 September 2015, http://www.38north.org.
 Jeffrey Lewis, “Recent Imagery Suggests Increased Uranium Production in North Korea,” 38 North, 12 August 2015, http://www.38north.org.
 «Kim Jong Un Visits Reconstructed Pyongchon Revolutionary Site,» Rodong Sinmun, 10 December 2015, rodong.rep.kp.
 «CTBTO Executive Secretary Lassina Zerbo on the unusual seismic event detected in the Democratic People’s Republic of Korea.» CTBTO Preparatory Commission. Last modified January 6, 2016. https://www.ctbto.org