The paper reviews the development of terminal insulation as used on capacitors over about three decades, and shows the gradual increase of appreciation of the need for sealing them against the ingress of moisture. Such sealing is needed in general to maintain their electrical properties unimpaired over long periods of time. Part of these improvements in sealing has evolved from the users' general need for improved quality, and part has arisen from the improvement in electrical characteristics of the capacitors themselves that has taken place over the same period, which has automatically necessitated the better sealing to retain those better characteristics. A classification of the various forms of sealing is suggested, which is partly chronological and partly based upon the form and nature of the sealing medium; while a Bibliography is included to substantiate the chronological aspect of the review.