Human Physiology is the English version of a time-honored German textbook first published by HERMANN REIN in 1936. We undertook the preparation of a completely revised 20th edition with the intention of making the book accessible to a wide range of English-speaking readers. The subject-matter was therefore organized so as to corre spond to the structuring of physiology courses in most countries of the world. The book is directed primarily at students of medicine. Its aim is to enable them to un derstand living processes in the human organism, providing the basis for the scientific understanding of pathological changes. The material was chosen to give the reader not only the knowledge required for passing examinations, but also information necessary for a subsequent professional career. For this reason special attention was devoted to pathophysiological aspects. We hope that the book will prove a useful reference on the present status of physiology for physicians in private and hospital practice as well as for its primary readership. The book should also serve biologists, biochemists, pharmacologists, pharmacists, and psy chologist as a source of information on the physiological principles underlying their disciplines.