The increase in transmission system voltages in recent years to 500 kV, with even higher prospective values, makes the internal insulation a large proportion of the transformer cost and therefore economically more important. The associated increase in MVA rating, and hence in size and weight, has reached beyond transport limits and demands better insulation of smaller volume. These factors can both be helped by increasing the working stresses in the insulation. The paper considers means of achieving this by improvements in impregnation, more uniform dielectric fields and the elimination of creepage, voids and local stresses due to winding connections and transpositions. Since oil-impregnated paper is exclusively used in high-voltage power-transformer windings, the elements paper and oil are first considered and then the combination. This depends entirely upon the purity of the elements, the completeness of the impregnation and the subsequent prevention of contamination in service. These are also discussed. Throughout the paper the possibilities of non-destructive testing are considered. Distinction is drawn between the orthodox ageing of insulation and the practical life of transformers in service. Technical literature on the subject, though extensive, is notable for its discordance. The paper endeavours to incorporate a distillation of this practical information with the author's judgment and practical experience in the hope that discussion will lead to explicit confirmation or amendment of the conclusions reached.