Since September 11, 2001, Congress has provided about $808 billion to the Department of Defense (DOD) for the Global War on Terrorism (GWOT) in addition to funding in DOD's base budget. Prior GAO reports have found DOD's reported GWOT cost data unreliable and found problems with transparency over certain costs. In response, DOD has made several changes to its cost-reporting procedures. Congress has shown interest in increasing the transparency of DOD's cost reporting and funding requests for GWOT. Under the Comptroller General's authority to conduct evaluations on his own initiative, GAO assessed (1) DOD's progress in improving the accuracy and reliability of its GWOT cost reporting, and (2) DOD's methodology for reporting GWOT costs by contingency operation. For this engagement, GAO analyzed GWOT cost data and applicable guidance, as well as DOD's corrective actions. GAO is recommending that DOD (1) establish a methodology for determining what portion of GWOT costs is attributable to Operation Iraqi Freedom versus Operation Enduring Freedom and (2) develop a plan and timetable for evaluating whether certain expenses are incremental and should continue to be funded outside of DOD's base budget.