This monograph is aimed at the practising hydraulic engineer. Work on it commenced at Professor Naudascher’s instigation in 1982. Over the next six years all or some of the authors discussed progress at IAHR sponsored conferences at Esslingen, Melbourne, Lausanne and Beijing. With the authors scattered throughout the world, and all with other responsibilities, progress was bound to be slow. Completion was further delayed by the great increase in published technical literature in this area over the period 1982-1988. This literature continues to expand and with it our understanding of the air water flow phenomena. The monograph must therefore be seen as the authors’ views on the state of the art around 1988. More recent references have been included for completeness.This monograph has been a joint effort with most authors making suggestions and contributions to more than one chapter. Nevertheless, the chapter authors are primarily responsible for the material in their chapters.Throughout the monograph symbols are defined when they are fist introduced and a list of symbols is included at the end of each chapter.Many other people have contributed to this monograph, but the authors would particularly like to acknowledge the assistance given by Professor John Mc Nown who has read, commented on and improved the style of the complete monograph.