The paper reviews briefly some of the characteristics of epoxy-resin systems that are of importance in their application to electrical components by encapsulation, impregnating, laminating, etc. The characteristic properties of these materials are those peculiar to their processing, i.e. low shrinkage and absence of volatile matter on cure. Characteristics important in their application, such as pot life, viscosity, exotherm and shrinkage, are therefore discussed. Some aspects of their electrical and mechanical properties are also discussed, and, though a detailed description of new material is outside the scope of the paper, present and future trends are briefly mentioned.