Although Iran claims that its nuclear program is exclusively for peaceful purposes, it has generated considerable concern that Tehran is pursuing a nuclear weapons program. Indeed, the UN Security Council has responded to Iran's refusal to suspend work on its uranium enrichment and heavy-water nuclear reactor programs by adopting several resolutions which imposed sanctions on Tehran. Despite this pressure, Iran continues to enrich uranium, install and operate additional centrifuges, and conduct research on new types of centrifuges. Tehran has also continued to produce centrifuge feedstock, as well as work on its heavy-water reactor and associated facilities. Whether Iran is pursuing a nuclear weapons program is, however, unknown. A National Intelligence Estimate made public in December 2007 assessed that Tehran "halted its nuclear weapons program," defined as "Iran's nuclear weapon design and weaponization work and covert uranium conversion-related and uranium enrichment-related work," in 2003. The estimate, however, also assessed that Tehran is "keeping open the option to develop nuclear weapons" and that any decision to end a nuclear weapons program is "inherently reversible." Intelligence community officials have reaffirmed this judgment on several occasions. Iranian efforts to produce fissile material for nuclear weapons by using its known nuclear facilities would almost certainly be detected by the IAEA. Although Iran has cooperated with the International Atomic Energy Agency (IAEA) to an extent, the agency says that Tehran has not gone far enough to alleviate all of the IAEA's concerns about Iran's enrichment and heavy-water reactor programs. The IAEA continues to investigate the program, particularly evidence that Tehran may have conducted procurement activities and research directly applicable to nuclear weapons development. This report expands and replaces CRS Report RS21592, Iran's Nuclear Program: Recent Developments, by Sharon Squassoni, and will be updated as necessary.