When considering the requirements of insulation for high-voltage equipment there are a number of factors which must be taken into account; some of them affect the long-term life of the insulation, and others affect its performance only under abnormal stress of very short duration. As the result of the increasing knowledge of the behaviour of power systems, the characteristics of the various forms of insulation used in the construction of high-voltage equipment and improved techniques in testing, it is now possible to introduce into apparatus specifications clauses which deal much more specifically with insulation requirements than in the past. Many specifications which are being reviewed at present are incorporating new requirements for the performance of insulation. It is the object of the paper to consider in general terms the system characteristics which affect the choice of insulation, to assess the various over-voltages which may be imposed upon apparatus, to consider the different forms of overvoltage protection which are available, and to offer guidance on the selection of insulation for the usual applications covered by national and international specifications.