Underground cables are generally installed out of sight and are expected to operate without maintenance for at least 40 years. They are likely to represent the most valuable asset in a utility distribution network. Basic cable design has changed little over more than a century since the first cables enabled the beginning of electricity distribution in the UK. The principal advances have been in materials and manufacturing processes, especially regarding insulation and protection. This has permitted the development of underground cables for ever-increasing system voltages through the 20th century. Impregnated paper retained its monopoly position as the primary insulation of all low and high voltage cables for the first half of the century, only gradually being replaced by polymeric materials newly developed for electrical applications. Cross-linked polymeric insulation has significant advantages over impregnated paper, not least being higher operating temperature allowing increased current rating for each conductor size. Early experience with cross-linked polyethylene cable insulation outside the UK was not all good, and this spurred research into the causes of premature failure and how to improve long-term reliability. Experience with polymeric cables in UK utility and industrial distribution networks is now approaching 40 years and the reliability record so far is good. National and international design and performance standards together with qualification testing regimes are well established.