Medium Voltage power cables are essential components of electricity grids, at the same time many cables have yet been more than 30 years in service and their condition can hardly be evaluated. Repairs and exchange of underground cables on the one hand always lead to high work effort, official procedures and costs. On the other hand, cable failures can lead to power outages and difficulties in security of supply. In the present thesis, an overview of the physics and ageing of cables and the current state of research to this topic is given. The practical main problem here is the gathering of data about the current cable condition, because relevant cableparameters can usually not be determined while in operation. Possibilities for an assessment of the actual cable condition are technical diagnostic techniques. The most widely used ones, measurements of partial discharges and the dissipation factor tan are explained with practical examples of measurements from the grid of Kärnten Netz GmbH. Finally, the current state of research of a promising method for using power line communication (PLC) signals for an online condition monitoring of cable grids is shown, analyzed and compared to the current methods.