The present book is the result of diverse courses on diffusion. It is intended to give as complete an overview as possible of diffusion in solid media, while relating the process of diffusion to both its physical basis and its applications. Several real situations are covered in this account, from self-diffusion of radiotracers to the more complex cases of mass flow under chemical or thermal gradients or under electric fields, or diffusion in structures of lower dimensionality (surfaces and interfaces). In all these analyses, no category of materials was favored; metals, ionic crystals, oxydes, and semiconductors all had their turn. Only polymers were not specifically touched. One chapter is specifically devoted to techniques for studying diffusion, including methods of numerical simulation, and the long last chapter gives a number of metallurgical phenomena in which diffusion plays a fundamental role.