High-intensity high-pressure mercury-vapour (h.p.m.v.) lamps are now being used in cinema studios and for other important applications. During the development of lamps and circuits suitable for these purposes, it was necessary to solve circuit difficulties of a rather special nature, and it is the purpose of the paper to discuss these problems. Fairly complete descriptions are also given of some of the circuits which experience has shown to be suitable for commercial use. Although most of the lamps towards which the discussion is directed are of the high-pressure mercury-vapour type, the principles are often of much wider interest and application.