In recent years, 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. Much of the debate surrounding passage of the 2012 NDAA centered on what appeared to be an effort to confirm or, as some observers view it, expand the detention authority that Congress implicitly granted the President via the AUMF in the aftermath of the terrorist attacks of September 11, 2001. But the 2012 NDAA addressed other issues as well, including the continued detention of persons at Guantanamo. Both the 2013 NDAA (P.L. 112-239) and the 2014 NDAA (P.L. 113-66) also contain subtitles addressing U.S. detention policy, though neither act addresses detention matters as comprehensively as did the 2012 NDAA. The FY2015 NDAA (P.L. 113-291) and the Consolidated and Further Continuing Appropriations Act, 2015 (2015 Cromnibus; P.L. 113-235), essentially maintain the status quo. The 2012 NDAA authorizes the detention of certain categories of persons and requires the military detention of a subset of them (subject to waiver); regulates status determinations for persons held pursuant to the AUMF; regulates periodic review proceedings concerning Guantanamo detainees; and continued funding restrictions on Guantanamo detainee transfers. During floor debate, significant attention centered on the extent to which the bill and existing law permit the military detention of U.S. citizens believed to be enemy belligerents, especially if arrested within the United States. The enacted version included a provision clarifying that the act's affirmation of detention authority under the AUMF is not intended to affect existing authorities relating to the detention of U.S. citizens or lawful resident aliens, or any other persons arrested in the United States. When signing the 2012 NDAA into law, President Obama stated that he would "not authorize the indefinite military detention without trial of American citizens." The 2012 NDAA and subsequent defense authorization enactments also included provisions concerning the transfer or release of detainees currently held at Guantanamo. Both the 2012 and 2013 NDAAs extended the existing prohibition on the release of detainees into the United States for any purpose, as well as restrictions upon the transfer of such Guantanamo detainees to foreign countries. The 2014 NDAA extends the blanket prohibition on transferring Guantanamo detainees to the United States, but allows the Executive greater flexibility in determining whether to transfer detainees to foreign custody. Both policies are continued in the 2015 NDAA and the 2015 Cromnibus, and the Obama Administration has stepped up the transfer of detainees to foreign countries. This report offers a brief background of the salient issues raised by the detainee provisions of the FY2012 NDAA, provides a section-by-section analysis, and discusses executive interpretation and implementation of the act's mandatory military detention provision. It also addresses detainee provisions in the 2013 NDAA and 2014 NDAA, as well as those considered during the House and Senate deliberations of the 2015 NDAA. An earlier version of this report was entitled The National Defense Authorization Act for FY2012 and Beyond: Detainee Matters.