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When energizing an electrical insulation system with a direct voltage the initial field distribution, being a capacitive field distribution free of space charges like under normal ac voltage stress, turns into a stationary resistive field distribution. Both, the stationary field distribution and the duration needed for the transition from the capacitive to the resistive field distribution are governed by the bulk and the surface conductivity of the insulating materials involved. Any change of the field distribution compared to the capacitive field is associated with the accumulation of space charges in the bulk material or surface charges on the interface between different insulating materials. Particularly for gaseous dielectrics a simulation model is presented, which takes into account the generation, recombination and motion of charge carriers. The field calculations using this model are verified by measurements of the surface potential on cylindrical epoxy resin insulators under low electric field stress. By means of the simulation model, the charging of conical insulators under high electric field stress and the influence of field-induced electron emission from the cathode is investigated. The influence of temperature and field strength on the surface and bulk conductivity of epoxy resin material is shown experimentally. The influence of temperature-dependent bulk conductivity on the resistive field distribution is shown using simulations. Current measurements in gas under high field conditions indicate the existence of a source of electric charges besides natural ionization. In order to achieve a faster transition from the initial capacitive field distribution to the stationary resistive field distribution (e.g. when an HVDC system is energized, or when a polarity reversal takes place) and in order to provide a faster decay of surface charge carriers, an increase of the insulators’ surface and bulk conductivity is recommended. For experimental investigations, the surface and bulk conductivity of conventional epoxy resin insulators were adjusted by functional fillers providing a well-defined conductivity of the compound. Aiming for eco-friendly solutions, the electric direct voltage strength of nitrogen, oxygen, and gas mixtures has been determined. Further, the influence of argon in such mixtures has been investigated. With respect to possible application in dc insulation systems the electric strength of three typical electrode arrangements with uniform, slightly and strongly non-uniform electric field has been determined experimentally and relevant values compared to the electric strength in SF6 are given.