The main properties required of paper for use in high-voltage dielectrics are concerned with its electric strength and dielectric losses. The effect of such physical properties as thickness, apparent density and air impermeability upon the impulse strength of impregnated paper are studied by both flat-sheet and model test methods, not only for types used in cables and transformers, but also for those used in capacitors.The incorporation of synthetic and other types of fibre into softwood pulp is discussed, and the electric strength is shown to be dependent upon the associated physical properties.Factors affecting dielectric losses, on the other hand, are the chemical constitution and purity of the pulp and water from which the paper is made, as well as its apparent density. The effect on the electrical properties of changes in the pulp and washing treatment is demonstrated, and the importance is shown in the case of capacitor tissues of the presence of conducting particles and other impurities. Methods of assessment of quality are described in detail.