Pursuant to a congressional request, GAO reviewed Department of Defense (DOD) requirements for its sealift and airlift mobility operations, focusing on whether key assumptions in the DOD mobility requirements study were reasonable. GAO found that: (1) DOD ability to move U.S. forces abroad is dependent upon key assumptions regarding the extent of the enemy threat and the amount of advanced warning; (2) increases in the amount of enemy threat have increased the amount of mobility forces required, while warning time assumptions have achieved a moderate-risk outcome; (3) the DOD mobility requirements study has understated U.S. sealift capabilities and is based on questionable future ship procurements; (4) the DOD sealift mobility study has failed to account for the use of foreign, commercial container, and Marine Corps prepositioning ships; (5) the DOD airlift mobility study has overstated U.S. airlift cargo capabilities and is based on unrealistic procurement, delivery schedule, and foreign air base availability assumptions; and (6) additional testing of key assumptions is needed to justify DOD mobility asset requirements.