Most water systems in the industrial regions of the world are already developed. At the same time they are highly complex. This is true with respect to physical configuration, managment, operation, political goals, environmental interactions, etc. Thus the basic systems are already in place. This realization is the starting point for any new water developments and for operation. From this we conclude that whatever we do to meet new exigencies requires an understanding of the presently in-place complex water systems. Their operation is the important thing. And how can we adjust their operation to meet the new demands upon the system? This book deals with complex water systems and their operation. Some chapters are highly theoretical while others are rooted in practical applications. How can we an~lyze the operation of a complex water system and determine how its performance can be improved? Several chapters on mathematical analysis give approaches involving different aspects of this problem. But operation also has political, management, and physical aspects. These problems are addressed in chapters by managers who operate such systems. The main theme of all chapters is how to deal with the different aspects of a complex water system, already in place. We feel the book, in dealing with this question could be a start for new theoretical premises in water planning.