This study was initiated to determine the in-service reliability, maintainability and availability of selected critical wastewater treatment plant components. Components were considered critical if their failure caused an immediate impact on effluent quality. Treatment plant components selected included pumps, power transmissions, motors, compressors, valves, and controls. Components were those from four types of wastewater treatment plants. The four types of plants were air activated sludge, oxygen activated sludge, trickling filter, and rotating biological contactor (RBC). Operation and maintenance data obtainde drom nine operating plants were used to estimate failure rates, mean-time-between-failures, mean-time-to-repair, and availability for each component. These performance statistics are by component type, size range, and application. Performance statistics were calculated from operating data for 119 pumps, 249 power transmissions, 285 motors, 17 compressors, 13 valves, and 269 controls. Calculated performance statistics can be utilized by design engineers and plant operators to assist in the selection of new equipment. The development of a performance data base and subsequent proper utilization of the data should result in improvements in treatment plant performance. These data allow estimates to be made of performance to be expected from certain equipment in certain applications.