This paper presents a review of published work on flashover, mainly of Russian, German and American origin. The information obtained from these sources, combined with supporting experimental work, enables a comprehensive explanation of the phenomena to be given. A list of running conditions for which flashover has been reported is given. There are two basically different causes of flashover; they are (a) severe brush arcing, and (b) interbar arcing caused by conducting debris etc. in the gaps. The voltage distribution around the commutator is then critical in determining whether flashover occurs when either of these conditions exists. Methods of avoiding or reducing the risk of flashover are suggested. A comprehensive list of references is included.