The approach of this treatise is physiological throughout. In the eyes of the author it answers the rhetorical question raised by Maurice B. Visscher at the Physiology Congress in Washington D. C. in 1968: Does physiology exist? What he meant by this question was whether the fields of cellular physiology and physiology of the various organ systems had become so large that physiology as such had vanished. The firm answer is that physiology does indeed exist. Although it is important to study physiological problems at the subcellular level, it is importan- and equally difficult - to study organ regulation at the cellular level, organ interaction, and integration into the whole organism. An account of avian osmoregulation from an integrated point of view is attempted in this book. Since reading Homer W. Smith's From Fish to Philosopher and August Krogh's Osmoregulation in Aquatic Animals verte brate osmoregulation has been in the center of the author's interest. The focus was set on avian osmoregulation after personal contact with the School of Krogh when working in the laboratory of Bodil M. Schmidt-Nielsen. The fundamental concepts and isotope techniques introduced by Hans H. Ussing have been of constant inspiration. An excellent example for the study of osmoregulation at the cellular level was given by the late Jean Maetz. The writing of this book was suggested by Donald S. Farner who is thanked for thorough editorial assistance, and especially with help in the subtle semantic peculiarities of the English language.