the technique, termed reactive sputtering, of preparing thin films of metallic compounds, notably metal oxides, by sputtering from metallic cathodes in the presence of active gases is described and compared with other deposition processes. The oxidation mechanisms which may occur during the sputtering of a metal in an atmosphere containing oxygen are considered. The desirable performance features of a reactive sputtering plant are enumerated. Experience of producing a range of metal oxide coatings has shown that in order to obtain films of a specified chemical composition the concentration of the desorbed gases in the sputtering atmosphere must be reduced to a minimum. This can be achieved by the use of a vapour pump, having a sufficiently high speed at the sputtering pressure to permit a high rate of leakage of the desired gas into the sputtering chamber. The composition of films of metal oxides which do not obey the law of fixed proportions can be varied by sputtering in oxygen plus inert gas mixtures of different concentrations. A simple method for controlling the composition of such gas mixtures is described. Consistent control of the sputtering rate requires the provision of a stable glow discharge operating at a given applied voltage and current density. It is shown that a booster vapour pump used in conjunction with a high vacuum isolation valve of variable aperture permits fine control of the sputtering pressure on which the discharge voltage and current are dependent, also, that a stable glow discharge can be easily obtained if the high tension transformer is of the high reactance type.