The Department of Justice (DOJ) estimates that the United States has about a million gang members. While state and local agencies have primary responsibility for combating gang crime, the federal government has key roles to enforce laws and help fund programs to provide alternatives to gang membership for at-risk youth. GAO was asked to examine federal efforts to combat gang crime. This report addresses (1) the roles of DOJ and the Department of Homeland Security (DHS) in combating gang crime and the extent to which DOJ and DHS agencies coordinate their efforts with each other and state and local agencies; (2) the extent to which DOJ and DHS measure their gang enforcement efforts; and (3) how federal grant funding is used to administer or support activities to reduce gang-related crime. GAO reviewed federal agencies' plans, resources, and measures and interviewed federal, state, and local officials in 15 localities with federally led anti-gang task forces representing varying population sizes and locations. GAO recommends that DOJ reexamine roles of headquarters anti-gang entities, establish performance measures for anti-gang efforts, review USAO gang-related case data, and, along with DHS, develop a common definition for "gang." DHS concurred with the recommendations; DOJ is considering whether to establish measures and a common definition and concurred with the other recommendations. Seven appendices are included: (1) Nature and Scope of Gang and Gang Crime in the United States; (2) Federal Statutes Used in Gang-Related Prosecutions; (3) Important Elements of an Approach to Combating Gangs; (4) Scope and Methodology; (5) Comments from the Department of Justice; (6) Comments from the Department of Homeland Security; and (7) GAO Contact and Staff Acknowledgments. (Contains 4 tables and 12 figures.)