The phenomenon of evaporation in the natural environment is of interest in various diverse disciplines. This book is an attempt to present a coherent and organized introduction to theoretical concepts and relationships useful in analyzing this phe nomenon, and to give an outline of their history and their application. The main objective is to provide a better understanding of evaporation, and to connect some of the approaches and paradigms, that have been developed in different disciplines concerned with this phenomenon. The book is intended for professional scientists and engineers, who are active in hydrology, meteorology, agronomy, oceanography, climatology and related environ mental fields, and who wish to study prevailing concepts on evaporation. At the same time, I hope that the book will be useful to workers in fluid dynamics, who want to become acquainted with applications to an important and interesting natural phenomenon. As suggested in its subtitle, the book consists of three major parts. The first, consisting of Chapters I and 2, gives a general ouline of the problem and a history of the theories of evaporation from ancient times through the end of the nineteenth century. This history is far from exhaustive, but it sket~hes the background and the ideas that led directly to the scientific revolution in Europe and, ultimately, to our present-day knowledge.