The interfacial reactions and microstructures of metal-GaAs contacts are, in general, much more complicated and difficult to control than the corresponding metal-Si contacts. They are very sensitive to reaction temperature, ambient, metal layer thickness, and the GaAs surface cleaning procedure. Many of the ohmic and gate contacts to GaAs currently in use or under development for GaAs FET devices are comprised of more than one metal species and in some cases a doping element as well. In dealing with these complexities, information about the microstructure at the contact interface is critically needed: for the evaluation of a specific contact metallurgy, for the definition of an optimum fabrication process, and, most important of all, for generating new ideas for better contact schemes. In this paper, our TEM and STEM studies of several ohmic and gate contacts that are of technological interest will be described. Attention will be drawn to the link between the interfacial microstructures and their electrical behavior, the kinetics of interfacial reactions, and thermal stability. The current constraints on obtaining ideal, reliable and controllable metal-GaAs contacts will also be discussed.