I n the years since President Obama took office, Congress has included provisions in annual defense authorization bills addressing issues related to detainees at the U.S. Naval Station at Guantanamo Bay, Cuba, and, more broadly, the disposition of persons captured in the course of hostilities against Al Qaeda and associated forces. The National Defense Authorization Act for FY2012 (2012 NDAA; P.L. 112-81 ) arguably constituted the most significant legislation informing wartime detention policy since the 2001 Authorization for the Use of Military Force (AUMF; P.L. 107-40 ), which serves as the primary legal authority for U.S. operations against Al Qaeda and associated forces. Both the National Defense Authorization Acts for FY2013 (2013 NDAA; P.L. 112-239 ) and FY2014 (2014 NDAA; P.L. 113-66 ) contain subtitles addressing U.S. detention policy, particularly with respect to persons held at Guantanamo, though neither act addresses detention matters as comprehensively as the 2012 NDAA. The FY2015 NDAA ( P.L. 113-291 ) essentially maintained the status quo established by the 2014 NDAA, while the FY2016 NDAA ( P.L. 114-92 ) reverted to a version of the earlier policy. During congressional deliberations over the House and Senate bills competing to become the 2012 NDAA, the White House criticized each version's detainee provisions, and threatened to veto any legislation "that challenges or constrains the President's critical authorities to collect intelligence, incapacitate dangerous terrorists, and protect the Nation." In particular, the Administration expressed strong opposition to any provision mandating the military detention of certain categories of persons, limiting executive discretion as to the appropriate forum to prosecute terrorist suspects, or constraining the Executive's ability to transfer detainees from U.S. custody. The version of the 2012 NDAA passed by Congress included a few modifications intended to assuage some of the Administration's concerns. The conference report dropped a House provision that would have required military commissions for certain terrorism cases and modified the House provision prohibiting the transfer of terrorism suspects to the United States for trial so that it only applies to those held at Guantanamo and not to all suspects detained abroad. It modified the Senate provision mandating the military detention of certain categories of persons (originally subject to waiver by the Secretary of Defense) by adding a statement to that provision to confirm that it does not affect "the existing criminal enforcement and national security authorities of the Federal Bureau of Investigation or any other domestic law enforcement agency," even with respect to persons held in military custody. The conferees also transferred the waiver authority from the Secretary of Defense to the President. The conference report retained language added during Senate floor debate to clarify that the provision affirming the authority to detain persons captured in the conflict with Al Qaeda does not modify any existing authorities relating to the power to detain U.S. citizens or lawful resident aliens, or any other persons captured or arrested in the United States. The Obama Administration then lifted its veto threat, and President Obama signed the 2012 NDAA into law on December 31, 2011. Nonetheless, President Obama issued a signing statement criticizing many of the act's detainee provisions, in which he pledged to interpret certain provisions in a manner that would preserve a maximum degree of flexibility and discretion in the handling of captured terrorists. Among other things, he criticized the blanket bar on Guantanamo detainee transfers into the United States and the restrictions imposed on detainee transfers to foreign countries, arguing that some applications of these provisions might violate constitutional separation of powers principles. President Obama also announced that he would "not authorize the indefinite military detention without trial of American citizens," regardless of whether such detention might be legally permissible under the AUMF or the 2012 NDAA. He further declared that his Administration would not "adhere to a rigid across-the-board requirement for military detention," and suggested that he would exercise the statutory waiver of the mandatory military detention provision when he deemed it appropriate. On February 28, 2012, President Obama issued a directive describing circumstances in which the 2012 NDAA's mandatory military detention requirement would be waived. Both the 2013 and 2014 versions of the NDAA contain subtitles addressing U.S. detention policy, particularly with respect to persons held at Guantanamo. While the detention provisions in the 2013 NDAA largely represented a continuation of existing policies, the 2014 NDAA saw some relaxation of the long-standing restrictions imposed upon the transfer of Guantanamo detainees to foreign countries. The 2015 NDAA did not modify the requirements established by the 2014 NDAA. The Consolidated and Further Continuing Appropriations Act, 2015 (2015 Cromnibus, P.L. 113-235 ), extended through FY2015 the prohibition on the use of funds to transfer detainees abroad unless the requirements of the 2014 NDAA are satisfied. The Obama Administration subsequently stepped up transfers of cleared detainees to foreign countries. The Administration's noncompliance with a congressional notification requirement in the 2014 NDAA, in connection with the transfer of five Taliban detainees in exchange for U.S. Army Sergeant Bowe Bergdahl in 2014, appears to have swayed some in Congress toward reinstating and strengthening the previous set of requirements for transferring detainees to foreign countries. The 2016 NDAA reenacted a modified version of earlier restrictions and imposed some new ones. This report offers a brief background of the salient issues and provides a section-by-section analysis of the detainee provisions in the National Defense Authorization Act for FY2012. It also discusses executive interpretation and implementation of the act's mandatory military detention provision. Finally, it addresses detainee provisions in subsequent national authorization legislation and other developments. At the heart of the consideration of the detainee provisions in the 2012 NDAA appears to have been an effort to confirm or, as some observers view it, expand the detention authority Congress implicitly granted the President in the aftermath of the terrorist attacks of September 11, 2001. In enacting the Authorization for Use of Military Force (AUMF; P.L. 107-40 ), Congress authorized the President to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons. Many persons captured during subsequent U.S operations in Afghanistan and elsewhere have been placed in preventive detention to stop them from participating in hostilities or terrorist activities. A few have been tried by military commission for crimes associated with those hostilities, while many others have been tried for terrorism-related crimes in civilian court. In the 2004 case of Hamdi v. Rumsfeld , a majority of the Supreme Court recognized that, as a necessary incident to the AUMF, the President may detain enemy combatants captured while fighting U.S. forces in Afghanistan (including U.S. citizens), and potentially hold such persons for the duration of hostilities. The Hamdi decision left to lower courts the task of defining the scope of detention authority conferred by the AUMF, including whether the authorization permits the detention of members or supporters of Al Qaeda, the Taliban, or other groups who are apprehended away from the Afghan zone of combat. Most subsequent judicial activity concerning U.S. detention policy has occurred in the D.C. Circuit, where courts have considered numerous habeas petitions by Guantanamo detainees challenging the legality of their detention. Rulings by the U.S. Court of Appeals for the D.C. Circuit have generally been favorable to the legal position advanced by the government regarding the scope of its detention authority under the AUMF. It remains to be seen whether any of these rulings will be reviewed by the Supreme Court and, if such review occurs, whether the Court will endorse or reject the circuit court's understanding of the AUMF and the scope of detention authority it confers. Prior to the 2012 NDAA, Congress did not pass any legislation to directly assist the courts in defining the scope of detention authority granted by the AUMF. The D.C. Circuit has, however, looked to other post-AUMF legislation concerning the jurisdiction of military commissions for guidance as to the categories of persons who may be subject to military detention. In 2010, the circuit court concluded that the government had authority under the AUMF to detain militarily persons subject to the jurisdiction of military commissions established pursuant to the Military Commissions Acts of 2006 and 2009 (MCA); namely, those who are "part of forces associated with Al Qaeda or the Taliban," along with "those who purposefully and materially support such forces in hostilities against U.S. Coalition partners." Most of the persons detained under the authority of the AUMF are combatants picked up during military operations in Afghanistan or arrested elsewhere abroad. Many of these individuals were transported to the U.S. Naval Station at Guantanamo Bay, Cuba, for detention in military custody, although a few "high value" Guantanamo detainees were initially held at other locations by the CIA for interrogation. The United States also held a larger number of detainees at a facility in Parwan, Afghanistan. The United States agreed to hand over control of the facility to the Afghan government, and transfer all Afghan detainees in its custody at Parwan to the Afghan authorities by September 2012, though implementation of this agreement was delayed due to disagreement between Afghan and U.S. authorities, with Afghan officials reportedly claiming that the United States continued to hold several dozen Afghan detainees. The remaining Afghan prisoners were turned over to Afghanistan in March 2013 after an agreement was reached whereby U.S. advisors are to remain at the facility and Afghanistan agreed not to release prisoners the United States considers particularly dangerous.