This book is a second edition of the one that was published by John Wiley & Sons in 1988. The author's intention remains the same in this edition, which is to teach basic aspects of, and methods of solution for diffusion phenomena through physical examples. The emphasis is on modeling and methodology. This book seeks to bridge the gap between physico-chemical statements of certain kinetic processes and their reduction to diffusion problems. It should also constitute a consistent introduction to diffusion phenomena, whatever their origin or further application.