In 1984, the U.S. Environmental Protection Agency (EPA) sponsored a study to evaluate the effectiveness and reliability of biological processes that depend on microorganisms to remove phosphorus. This study focused on field investigations at four full-scale wastewater treatment plants. Key objectives of the evaluation were to obtain information about the design, operation, and performance of biological phosphorus removal technologies; to document achievable effluent phosphorus concentrations; and to compare performance of the technologies based on full-scale operating experience. The study began in the spring of 1984 and continued until August 1985.