The contact properties between different metals and hydrogenated amorphous silicon, prepared by various deposition techniques in different laboratories, are reviewed. From these studies the appropriate metallizations have been established for the achievement of Schottky diode, quasi-ohmic or ohmic contact to undoped and doped films. The various characteristic parameters describing Schottky barrier interfaces such as ideality factor, current saturation, contact resistance and barrier height are discussed. The dependence of Schottky barrier height upon the metal work function, measuring and annealing temperature, and optical band-gap are also reported. The minority-carrier injection and series resistance effects on the contact properties of a-Si:H diodes are described. All the results are interpreted in terms of a self-consistent model that exhibits an electrode-limited to bulk-limited transition.