Why GAO Did This Study. Counterfeit parts--generally the misrepresentation of parts' identity or pedigree--can seriously disrupt the Department of Defense (DOD) supply chain, harm weapon systems integrity, and endanger troops' lives. In a November testimony (GAO-12-213T), GAO summarized preliminary observations from its investigation into the purchase and authenticity testing of selected, military-grade electronic parts that may enter the DOD supply chain. As requested, this report presents GAOs final findings on this issue. The results are based on a nongeneralizable sample and cannot be used to make inferences about the extent to which parts are being counterfeited. GAO created a fictitious company and gained membership to two Internet platforms providing access to vendors selling military-grade electronic parts. GAO requested quotes from numerous vendors to purchase a total of 16 parts from three categories: (1) authentic part numbers for obsolete and rare parts; (2) authentic part numbers with postproduction date codes (date codes after the last date the part was manufactured); and (3) bogus, or fictitious, part numbers that are not associated with any authentic parts. To determine whether the parts received were counterfeit, GAO contracted with a qualified, independent testing lab for full component authentication analysis of the first two categories of parts, but not the third (bogus) category. Part numbers have been altered for reporting purposes.