Among the challenges to mankind, few are more critical than the need to protect the environment. The rapid increase in population, coupled with the enormous rate on industrialization had a negative effect on the realization of this goal. As a result, the environ­ ment (water, air and earth) has been deteriorating more and more every day. It is only with the proper treatment of the wastes which are produced by man and his activities, that this deteriora­ tion can be stopped. Wastewater, is a major polluter of the environment. Although in many areas, science and technology has reached a level capable of preventing pollution, the reduction has not been realized for two reasons: (a) Lack of communication and transfer of knowledge to the desired extend between engineers and scientists, (b) Economic reasons. Good knowledge of the Biological Wastewater Treatment pro­ cesses is essential to overcome the economic handicaps. Because of that the improvement and dissemination of knowledge in this field was selected as the goal of the NATO - Advanced Study Insti­ tute held in Istanbul in July 1976. The lectures presented at this meeting have been compiled in the present volume.