Thousands of gang members in the United States belong to gangs such as MS-13 and 18th Street that are also active in Central American countries. Federal entities with responsibilities for addressing Central American gangs include the National Security Council (NSC); the Departments of Homeland Security (DHS), Justice (DOJ), and State; and the U.S. Agency for International Development (USAID). GAO was asked to review federal efforts to combat transnational gangs. This report addresses (1) the extent to which the federal government has developed a strategy to combat these gangs, and (2) how federal agencies have implemented the strategy and other programs to combat these gangs, coordinated their actions, and assessed their results. GAO examined federal agencies' antigang plans, resources, and measures; interviewed federal, state, and local officials in seven localities representing varying population sizes and geographic regions; and interviewed U.S. and foreign officials in El Salvador and Guatemala where U.S. agencies have implemented antigang programs. The results of these interviews are not generalizable. GAO recommends that the NSC revise the antigang strategy to include an approach for oversight and performance measures and that DOJ and DHS reach agreement on the composition of an antigang unit in El Salvador. The NSC did not comment. DOJ and DHS agreed with the author's recommendation to them. Nine appendices include: (1) Leadership Structure and Key Transnational Criminal Activities of MS-13 and 18th Street; (2) Characteristics of Other Transnational Gangs; (3) Federal Agencies' Programs to Connections to Central America Address Transnational Gangs with Connections to Central America; (4) Federal Agencies' Entities and Antigang Efforts Mechanisms for Coordinating Transnational Antigang Efforts; (5) Scope and Methodology; (6) Comments from the Department of Homeland Security; (7) Comments from the Department of Justice; (8) Comments from the United States Agency for International Development; and (9) GAO Contact and Staff Acknowledgments. (Contains 5 tables, 3 figures and 44 footnotes.)