The study summarized here 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. Eight treatment plant components were selected for study: pumps, power transmissions, motors, compressors, diffusers, valves, controls, and conveyors. These components were from four types of wastewater treatment plants: air activated sludge, oxygen activated sludge, trickling filter, and rotating biological contactor (RBC). Operation and maintenance data obtained from nine operating plants were used to estimate failure rates, mean time between failures (MTBF), mean time to repair (MTTR), 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. Design engineers and plant operators can use these calculated performance statistics to assist in selecting new equipment. The development of a performance data base and subsequent proper use of the data should improve treatment plant performance. These data allow the performance expected from certain equipment in certain applications to be estimated.