This book is the first comprehensive treatise of one of the key physical processes occurring in various materials at elevated temperatures. The book provides essential background information for materials scientists, metallurgists, solid state physicists and semiconductor technologists carrying out research or development in this and related areas. The first and second editions of the book were published by the University of Stuttgart in 1988 and 1989. In the present third edition the book has been updated and essentially enlarged to cover all recent developments in the area of grain and interphase boundary diffusion. The reader will find more than 100 new text pages, 60 new figures and 100 new references. This unique book is strongly recommended as a textbook for students as well as a reference book for physicists, chemists, metallurgists and engineers.