Today plastic-insulated cables, e.g. polyethylene (PE) and cross-linked polyethylene (XLPE), are being used more and more in the networks of utility companies. In contrast to paper-insulated mass-impregnated cables, this type of cable offers many advantages. But it also shows different ageing phenomena compared to the paper-insulated type, which requires a change of cable test procedures. Therefore in addition to voltage test methods, non-destructive test methods are necessary to get more information about the state of insulation. Research and development activities in co-operation with industry and utility companies are carried out in this field. Different kinds of diagnostic test methods, mainly in the time domain, are available or under development for the assessment of the quality of plastic-insulation.