Counterfeit parts, generally those whose sources knowingly misrepresent the parts' identity or pedigree, have the potential to seriously disrupt the Department of Defense (DOD) supply chain, delay missions, and affect the integrity of weapon systems. Almost anything is at risk of being counterfeited, from fasteners used on aircraft to electronics used on missile guidance systems. Further, there can be many sources of counterfeit parts as DOD draws from a large network of global suppliers. Based on a congressional request, GAO examined (1) DOD's knowledge of counterfeit parts in its supply chain, (2) DOD processes to detect and prevent counterfeit parts, and (3) commercial initiatives to mitigate the risk of counterfeit parts. GAO's findings are based on an examination of DOD regulations, guidance, and databases used to track deficient parts, as well as a Department of Commerce study on counterfeit parts; interviews with Commerce, DOD, and commercial-sector officials at selected locations; and a review of planned and existing efforts for counterfeit-part mitigation. GAO recommends that DOD leverage existing initiatives to establish anticounterfeiting guidance and disseminate this guidance to all DOD components and defense contractors. DOD concurred with each of the recommendations.