Since 1959 when P. Duwez showed at Caltech that Au-Si alloys could be quenched into a glassy state, there has been much interest in elucidating the nature of these amorphous materials. Certainly part of the motivation for studying amorphous alloys derives from their potential technological value: they are characterized by high hardness, corrosion and oxidation resistance, and high magnetic permeabilites and electrical resistivities but an equally strong motivation is simply that they are interesting materials. Scientific curiosity is stimulated by such fundamental questions as: What is their structure? Can defects be defined within this structure? What are the possible mechanisms of atomic transport? In addition, amorphous alloys provide a paradigm of a dense metastable structure, and how atomic transport can take place in such systems without transforming to more stable configurations is not well understood. Yet, as materials formed by nonequilibrium processing, e.g., nanocrystals, superlattices, rapidly solidified and ion-beam-modified materials, etc., find their way into technology, this question becomes increasingly germane. Ironically, much of the renewed interest in diffusion in amorphous alloys has been stimulated by the discovery, also at Caltech, of the solid-state amorphizing reaction, where multilayers of crystalline metal films transform to amorphous alloys by solid-state diffusion, rather than vice versa.