The paper deals with the problem of ageing (deterioration of insulating and mechanical properties) of extruded cable insulation. The today’s electric-power distribution networks are almost entirely fitted with the cables with extruded insulation. The expected life-time of the cables within the networks is 40 years. Depending on the inner and external electrical, mechanical, chemical, heat and other influences, this life-time can be less or more than 40 years. The first capital describes the processes initiated by theses influences, and the final result of these processes is the insulation ageing. The medium-voltage cables with extruded insulation have been mass-used for over 40 years, and due to the observations of the ageing and the break-down of the cables, some of the causes of the insulation ageing (water trees) are well explored and reduced by introduction of new cable materials and constructions. The impact of each individual influence (electrical, chemical, heat, mechanical and other) upon insulation in nowadays well explained in the technical literature, but their common simultaneous impact, which frequently occurs in the plant, isn’t fully explained yet. The second capital gives a survey of the finished cables testing, before and during the laying, as prescribed by the international standards. The tests serve for checking of the cable properties which are essential for the life-time of cables. Considering the fact that the operation conditions and the external conditions at the cable line change over the decades of usage, it isn’t possible to reach a decision concerning the state of the cable insulation by a mere monitoring of the influences (the monitoring doesn’t exist at the majority of the cable lines). A number of measurements has been developed in the world with the purpose of determination of the grade of insulation damage caused by ageing and the remaining life-time of the cables. The third capital brings the description of these measurements and the evaluation of the insulation damage based upon these measurements. The last capital brings a survey of the impact which the operating temperature has on the life-time of the cables. It presents the tests which have been carried out in the world, and subsequently it gives the description of the laboratory test made at the samples of the cross-linked polyethylene insulation.