A study of the ZnSe(100) surface and of its interfaces with Al and Au is presented. We find that the (2 X 1) reconstructed Se-rich surface is terminated with a full monolayer of dimerized Se, whereas the C(2X2) reconstructed Zn-rich surface corresponds to a half-monolayer of nondimerized Zn atoms. These atomic configurations and corresponding surface electron affinities are consistent with the requirement of dangling bond saturation and fully accounted for by the electron counting rule. For the metal/ZnSe interfaces, we find that Au forms an abrupt junction, whereas Al reacts and forms Al2Se3, releasing Zn in the process. The stabilized Fermi level position is 2.1 eV above the valence band maximum for Al and 1.15 eV for Au, irrespective of doping type and in qualitative agreement with the Schottky limit for the barrier heights. The Au/p-ZnSe Schottky barrier height can be reduced by 0.25 eV by introducing a 2-3 ML Se interlayer between the metal and the semiconductor.