Abstract: An apparatus for coating by sputter ion plating comprises a chamber having a gas inlet and outlet for passing gas under low pressure continuously therethrough; a cathode fabricated of coating material or precursor therefor positioned in the chamber; and means for generating a glow discharge within the chamber to sputter cathode material to form a coating thereof or of a product obtained therefrom on a substrate also positioned in the chamber.The cathode includes a plurality of projections (e.g. fins) extending from a surface, their disposition and geometry being such as to increase the effective ion current density in the chamber. In this way, high currents at low voltages and pressures may be achieved so that the apparatus can be used to coat a large substrate load, possibly at a higher coating rate.