The paper deals primarily with bushings of the synthetic-resin-bonded paper (s.r.b.p.) type, but many of the factors considered are common to other types.The characteristics required of bushings, and consequently the tests to which they are subjected, are dependent on the conditions of voltage, current, temperature and humidity under which they must operate. Various tests, their objects and limitations and their relation to service conditions are discussed, and a series of over-voltage and quality-control tests suitable for specification purposes are suggested.Finally some methods of site testing and the interpretation of results are discussed, illustrated by a number of service experiences. A method of power-factor testing on load is described.