As requirements for high quality wastewater effluent increase, quantities of sludge generated also increase. Contaminants removed from wastewater concentrate in the sludge and can represent serious environmental insult and health effects if not properly managed. The cost of sludge handling can be estimated to constitute as much as 50% of the cost of wastewater treatment. If properly managed, however, sludge may be considered a resource of value. Unfortunately, very little attention in the scientific commu nity has been given the sludge dilemma either in research or in practice. Sludge properties are presently characterized in various ways which do not permit rational design and operation of sludge management systems. The need for basic information regarding sludge properties and behavior was the impetus for a NATO Advanced Study Institute. The NATO Advanced Study Institute entitled "Sludge Character istics and Behavior" was held July 17-26, 1979, at the Uni versi ty of Delaware, Newark, DE 19711, USA. The excellent program quality was a result of both outstanding lecturers and participants. Twelve countries were represented at the Institute so that both formal, professional discussions and informal social activities crossed a broad spectrum of cultures. Gratitude is expressed to all contributors to this volume. Special acknowledgement must be given to the Scientific Affairs Division of the North Atlantic Treaty Organization. Without its financial support, this Advanced Study Institute would not have been possible.