A brief review of amorphous alloy formation by solid state reaction is presented from a kinetic viewpoint. We emphasize the kinetic barriers which govern the formation process. Specifically, the barrier to atomic diffusion and the barrier to nucleation are examined by using diffusion couples of amorphous NiZr alloys. Substantial interdiffusion can occur in the couple without crystallization, indicating that the barrier to nucleation of crystalline compounds is large so the amorphous alloy can be formed by a slow reaction. Submicron size voids in the amorphous alloy have been observed as a consequence of interdiffusion. The mechanism of diffusion in the amorphous alloy is discussed.