Formation of intermetallic phases upon heating of films composed of alternating layers of metal and amorphous silicon has been studied using power-compensated differential scanning calorimetry, crosssectional transmission electron microscopy, and thin film x-ray diffractrometry. Results for Ni/amorphous-Si (Ni/a-Si), Ti/a-Si, V/a- Si, and Co/a-Si are reviewed. In the first three cases, an amorphous silicide is the first phase to form. Further heating leads to thickening of the amorphous silicide and eventually to formation and growth of layers of crystalline silicides. In the case of Co/a-Si multilayer films, a crystalline silicide (CoSi) appears to be the first phase to form. In these systems calorimetric measurements suggest that there are barriers to nucleation of the crystalline phases, even though the energy reduction that would accompany their formation from pure components is large. It is suggested that interdiffusion may precede the formation of new phases at the original metal/a-Si interfaces, resulting in a significant decrease in the driving force for nucleation of the crystalline phases.