The diffusion or migration of atoms in matter, of whatever form, is a basic consequence of the existence of atoms. In metals, atomic diffusion has a well established position of importance as it is recognized that there are few metallurgical processes which do not embody the diffusion of one or more of the constituents. As regards semiconductors any thermal annealing treatment involves atomic diffusion. In semiconductor technology diffusion processes provide a vital and basic means of fabricating doped structures. Notwithstanding the importance of diffusion in the preparative processes of semiconductor structures and samples, the diffusion based aspects have acquired an empirical outlook verging almost on alchemy. The first attempt to present a systematic account of semiconductor diffusion processes was made by Boltaks [11 in 1961. During the decade since Boltaks' book appeared much work germane to understanding the atomic mechanisms responsible for diffusion in semiconductors has been published. The object of the present book is to give an account of, and to consolidate, present knowledge of semiconductor diffusion in terms of basic concepts of atomic migration in crystalline lattices. To this end, exhaustive compilations of empirical data have been avoided as these are available elsewhere [2, 31 : attention has been limited to considering evidence capable of yielding insight into the physical processes concerned in atomic diffusion.