This report responds to Congress's request that GAO determine the status and estimated costs of environmental cleanup on the island of Vieques. For decades, the U.S. Navy conducted ship-to-shore bombing exercises and other live-fire training activities on the island, which is located off the coast of Puerto Rico. The Navy ceased its operations on Vieques in 2003. The Navy has transferred the land to the Municipality of Vieques and the Puerto Rico Conservation Trust for conservation purposes and to the Department of the Interior. Although the land has been transferred, the Navy remains responsible for environmental cleanup. The cleanup is being carried out under the Defense Environmental Restoration Program (DERP) that consists of (1) the Installation Restoration Program, which addresses cleanup of hazardous substances, and (2) the Military Munitions Response Program, which addresses cleanup of munitions.