This chapter provides general remarks on diffusion in amorphous alloys. Amorphous alloys are thermodynamically not stable and may undergo structural transitions such as relaxation and crystallization during thermal annealing treatments. In the course of these transitions, usually controlled by diffusion processes, the extraordinary properties of the amorphous alloys are significantly changed or even destroyed by the process of thermal annealing. Diffusion studies on amorphous alloys can also be accompanied by structural transitions. Therefore, diffusion experiments in these materials are difficult to perform. The experiments are limited to very short diffusion lengths, often not more than about 10 nm, because the diffusion time at high temperatures is limited by the onset of crystallization, whereas at low temperatures the penetration is limited by the low diffusivity. An additional difficulty in the diffusion measurements arises from the change of the diffusivity in the amorphous state as a function of the annealing time if structural relaxation takes place. The diffusivity change can be significant in materials which have been produced by melt spinning and is shown in a figure. The reliability of individual sets of diffusion data depends here very much on the method of investigation. Unfortunately, in most of the diffusion investigations on amorphous alloys, the problems of structural transitions have not been noticed, or have been ignored. The latter obviously arises from an underestimation of its consequences for the diffusion results.