High voltage electrical cables play an immensely important, although largely unseen, part in the lives of everybody in the world today. They are mostly buried underground and provide trouble free operation for the majority of their operational lives. However, the polymer based insulation that is used for a large number of high voltage cables is subject to long term ageing which can eventually lead to electrical breakdown. This ageing manifests itself as the appearance of tree like structures in the bulk polymer insulator. The growth of a tree frequently starts on the boundary of the polymer at the so-called "polymer-semicon" interface. This thesis is concerned, however, with the changes that must take place in the polymer before the tree is formed. Previous investigations of field induced changes occurring within polymer insulation have involved cutting the polymer to expose the region of interest: this is not a satisfactory as the cutting process can produce changes in the polymer. To avoid this a novel technique was developed whereby the polymer-semicon interface can be exposed without cutting the polymer. The interface region of the polymer in contact with the plane electrode was examined and even though the field in this region is less than at the point, it can be sufficiently large to induce structural change in the polymer, readily detectable by Raman spectroscopy. In studies of both LDPE and XLPE, we find evidence of structural change within the polymer and most significantly of considerable Raman fluorescence which is indicative of defect states in the polymer. The latter becomes modified as the polymer structure approaches electrical failure. The observations reported are set in context by the examination of the work of relevant authors and some conclusions deduced. The evidence supports a model in which the forces induced by the electrical field lead to failure by the mechanisms of local yield, microvoid craze and crack formation commonly invoked for the mechanical fracture of polymeric solids.