Around 1990, the attention for subsea high-voltage direct current (HVDC) cables strongly increased especially due to the deployment of a large number of HVDC connections in the northwest of Europe. In Delft, this triggered the first research projects in the field of HVDC insulation. At that time, other industries also approached the high-voltage laboratory of Delft to start accumulating more knowledge in this field. In the early 1990s, Professor Kreuger led the high-voltage laboratory and one of his mottos was “DC does not exist”. This was used to motivate the students and make them aware of the fact that many different operating conditions are present under DC. The polarizations of the insulation and especially of the interfaces in the insulation are far from instantaneous, and the electric field in the insulation only gradually reaches a steady state. While a number of polarization and charging processes can be modelled, a more precise picture of the electric field distribution under different operating conditions is obtained by space charge measurements. This information is essential in the design process. For cable systems in operation, the proper knowledge of the electric and thermal stress distributions allows these systems to be operated more reliably and economically.