Dual water supply systems are two water supply distribution systems in a city, consisting of one fresh water system for potable use, and another system of seawater (or untreated raw fresh water, or treated/reclaimed wastewater) for toilet flushing purpose. In Hong Kong, dual water supply systems have been used for fifty years. The authors have done research projects in this area. The subject of the book is to discuss the use of dual water supply systems in their engineering and cost aspects.Fresh water is becoming a valuable and scarce resource nowadays. Any method or approach that can contribute to the saving of fresh water resource is good to our society and to the whole world. The book is suitable for use as a text book or reference book at undergraduate and postgraduate levels. University undergraduate students and postgraduate students in water science, civil engineering, environmental engineering and environmental science or management will be the principal audience. Practicing engineers, managers and other practitioners in these fields will also be potential audience for the book. The book is suitable for teaching, research and professional reference. There is no such book at all at present, and the authors think that it is badly needed in every part of the world.