The work of previous authors on the subject of paper as a cable-insulating material is briefly reviewed. It is shown that, in the past, attention has been directed too exclusively to the fibre stock, and the relation of the physical structure to the electrical properties has been relatively neglected. The characteristics which a cable-insulating paper should possess are then outlined. Briefly, these are high electric. strength, high oil-absorption, low electrical conductivity, high mechanical strength, and permanence.The results of tests designed to determine the relation of the paper structure to these qualities are then quoted and discussed. From these tests it is concluded that a satisfactory paper for power-cable insulation should have the fibres closely packed to give low bulk while still retaining high absorbency. The fibre packing should not be obtained by heavy surface calendering.The methods of test employed and the influence of the manufacturing processes on the product are briefly discussed in appendices.