Significant opportunities exist for improving the startup and long-term operating performance of effluent treatment systems in the pulp and paper industry. A review of performance limiting factors shows that different mills have similar problems. Performance problems never have a single cause, but rather, a combination of factors contribute to less than optimum performance. The correction of effluent treatment performance problems, therefore, requires a composite correction program that addresses each of the factors impacting plant performance. This paper discussed ten common causes of poor plant performance and lists twenty-one additional problems that are routinely encountered in the industry. The most frequently encountered problems are operational and design deficiencies, and these are revealed during startup or within the first few years of operation. Optimum plant performance can be expected when a design and startup plan addresses the performance limiting factors presented in this paper.