Several intermetallic compounds or ordered alloys (intermetallics) attract much attention as materials for high-temperature applications. This paper is devoted to our present knowledge of diffusion and its atomic mechanisms in some important binary intermetallics. Particular attention will be devoted to recent investigations mainly from our laboratory. These studies concern tracer diffusion in intermetallic compounds of the Ni-Al, Ti-Al, Fe-Al, and Fe-Si systems. Self-diffusion of components - the most basic diffusion process - is reviewed. Relevant factors for self-diffusion like the crystal structure, the state of order and disorder, the temperature and composition dependence are illustrated. In a few cases diffusion of selected foreign elements will also be considered.