Process control, as defined in this paper, includes all activities that result in the desired level of plant performance. Significant opportunities exist for improving the start-up and long-term operating performance of industrial activated sludge systems. A review of performance limiting factors shows that different plants have similar problems. Performance problems never have a single cause, but rather, a combination of factors contribute to less than optimum performance. An effective process control program identifies and removes these barriers to plant performance. This paper discusses ten common causes of poor plant performance and lists twenty-one additional problems routinely encountered in the industry. The most frequently encountered problems are operational and design deficiencies. Optimum plant performance can be expected when a design and an operating plan address the performance limiting factors presented in this paper.