The goals of wind wave research are relatively well defined: to be able to predict the wind wave field and its effect on the environment. That environment could be natural beaches, the atmosphere etc. or imposed by human endeavour ports, harbours, coastal settlements etc. . Although the goals are similar, the specific requirements of these various fields differ considerably. This book attempts to summarise the current state of this knowledge and to place this understanding into a common frame work. It attempts to take a balanced approach between the pragmatic engineering view of requiring a short term result and the scientific quest for detailed understanding. Thus, it attempts to provide a rigorous description of the physical processes involved as well as practical predictive tools.