In the past, industrial wastewater treatment primarily focused on the removal of BOD and suspended solids. In recent years, however, the focus has changed to aquatic toxicity, priority pollutants, and volatile organics. This required changes in how we design and operate biological treatment plants. Many existing plants must be retrofitted. New approaches to meet new requirements are discussed in detail. The authors, with a combined experience of sixty years, have presented case studies for a wide variety of industrial wastewaters including pulp and paper, food processing, chemical and pharmaceuticals, and textile wastewaters. Data interpretation and process design are developed through the use of seventeen examples. Procedures for the laboratory and pilot plant generation of process design data are presented. Emphasis is placed on meeting the many new regulations governing industrial wastewater discharges.