Waste water treatment is the largest biotechnology industry in the world, handling and disposing of domestic and industrial wastes so they present no threat to the general populace or the environment at large. The activated sludge process is the cornerstone of sewage treatment systems. Although it is a biological process and has been in use for more than 80 years, we still lack detailed understanding of how it works and how its performance might be better controlled and manipulated. This book presents a comprehensive review of what is currently known about the general microbiology of activated sludge and some of the bacteria responsible for the major operational problems encountered. Current understanding and the existing literature are critically assessed and new potentially valuable areas for research suggested. Methodology particularly appropriate to the study of the bacteria that cause bulking and foaming is described, and there is an identification section consisting of photomicrographs and detailed descriptions of the filamentous and other bacteria commonly seen in activated sludge plants. The impact of molecular methods on our understanding of activated sludge microbiology is emphasized throughout the book. The book will be of immediate interest to both microbiologists and waste water engineers, and has been written so as to be relevant and understandable to both. It will also be of interest and value to postgraduate students and researchers working in the fields of environmental engineering, general microbiology and microbial ecology.