This book has been a long time in preparation. Initially it grew out of our frustrating attempts over the past ten years to identify the filamentous bacteria seen in large numbers in most activated sludge plants, and the realization that we know very little about them and the other microbial populations in these systems. Unfortunately this book does not provide many answers to the problems these filamentous bacteria can cause, but we hope it might encourage microbiologists and engineers to communi cate more with each other and to spend some time trying to understand the tax onomy, ecology and physiology of activated sludge microbes. It is now very timely, for example, to try to provide these filamentous bacteria with proper taxonomically valid names and to determine their correct place in bacterial classifications. This book is not meant to compete directly with the books by Gray (1989, 1990) nor the excellent manual published by Jenkins and coworkers (1993b), which has been invaluable to us and others trying to identify filamentous bacteria. Wanner's book (1994a) also provides an excellent account of the problems of bulking and foaming caused by filamentous bacteria. These publications and others by Eikelboom's group have made an enormous contribution to the study of filamentous bacteria, and will con tinue to do so.