Early work evaluating toxicity reduction in municipal treatment plants revealed substantive ecosystem toxicity and Ames mutagenicity in plant effluents. The ecosystem toxicity in one plant effluent, where the plant flow contributed a substantial fraction of the stream flow, produced significant damage to the stream ecosystem. The evolving management method to control toxic and toxicity in wastewater treatment is called a Toxicity Reduction Evaluation (TRE). The TRE involves four major elements: evaluating the causes of toxicity pass through at treatment plants; characterizing and identifying the components of toxicity; tracing toxicity to its sources; and evaluating treatment alternatives for controlling toxicity. Progress on municipal case history TRE studies described in the paper indicate probable technical success in developing the TRE method for management ecosystem of toxicity in municipal wastewater treatment. While the relationship between the Ames mutagenicity and health effects risk has not been established, the relatively high levels of Ames mutagenicity in some municipal wastewaters suggest that health effects toxicity will be important in future TRE methods.