This book contains the lectures given in the International Course "Improving efficiency and reliability in water supply systems", hosted and sponsored by the Menendez Pelayo International University (U.I.M.P.) and co-sponsored by Aguas de Valencia, the British Council and the EC Cornett and Erasmus programmes. The short course took place in Valencia (Spain) in November 1994, with an attendance of more than one hundred delegates. We must not only acknowedge and thank Dr. Joaquin Azagra, as UIMP Director, but also his collaborators D. Luis Moreno and Lidia Lopez for their support in the preparation of the Course and during the course taking place. UIMP sponsorship allowed us to assemble in Valencia an eminent cadre of lecturers coming from all over the world, that covered in an ordered and precise fashion some of the more relevant aspects on efficiency and reliability in water supply systems. We are very thankful to all these leading lecturers for their invaluable cooperation. The publication of this book and the Spanish edition as well, have been made possible thanks to the sponsorship of both Polytechnic University of Valencia througout its Chancellor, Justo Nieto, and Aguas de Valencia throughout its General Director Alvaro Aguirre. We must also thank Kluwer Academic Publishers and especially their Publisher Petra van Steenbergen for her assistance, careful presentation and production of the book.