The paper describes those characteristics of lightning surges and system insulation and those electrical characteristics of systems which influence the design of surge diverters consisting of spark-gaps and non-linear resistors.The design of representative diverters of this kind is described and proposals for a British Standard for Surge Diverters are put forward.Methods of making and evaluating tests upon surge diverters are next considered, and details of the circuits and adjustment of the test equipment are given. The paper concludes with Sections on the installation and maintenance of surge diverters and upon surge counters.