In March of 1999, the Waste Isolation Pilot Plant (WIPP) in southeast New Mexico, the world's first deep geological repository for radioactive materials, began receiving defense-related transuranic waste. The WIPP was designed and constructed by the U.S. Department of Energy, but critical to its opening was certification by the U.S. Environmental Protection Agency that the repository complies with the radioactive waste disposal regulations set forth as environmental radiation protection standards (40 CFR Part 191) and compliance criteria (40 CFR Part 194). This paper provides a summary of the regulatory process, including the Environmental Protection Agency's waste containment, groundwater protection, and individual dose regulations for the WIPP; the Department of Energy's performance assessment and the other parts of its compliance certification application; and the Environmental Protection Agency's review and analysis of the compliance certification application and related documentation.