DHS had approximately 29,000 employees earning AUO, a type of premium pay intended to compensate eligible employees for substantial amounts of irregular, unscheduled overtime. DHS components' use of AUO has been a long-standing issue since at least 2007, when reviews identified the inappropriate use of AUO in DHS. GAO was asked to review DHS components' use and implementation of AUO. This report addresses, among other things, how much DHS spent on AUO from fiscal year 2008 through March 2014 (the most current data available) and the extent to which DHS components implemented AUO appropriately. GAO analyzed AUO payments data from components that have regularly used AUO, which included U.S. Customs and Border Protection, U.S. Immigration and Customs Enforcement, U.S. Secret Service, National Protection and Programs Directorate, U.S. Citizenship and Immigration Services, and Office of the Chief Security Officer. When calculating annual averages, GAO used the last full fiscal year of available data (2013). GAO also analyzed component AUO policies and procedures to assess compliance with federal regulations and guidance. GAO recommends that DHS develop and execute a department-wide mechanism to ensure components implement AUO appropriately. Congress should consider requiring DHS to report annually on components' progress remediating AUO implementation deficiencies.