The strength of electrical insulation in gaseous compressed SF6 is determined by various parameters, like pressure, polarity, field homogeneity, surface roughness of the electrodes, particulate contamination, and voltage wave shape. The dependence of the involved physical phenomena on accessible design parameters, like e.g. pressure, geometry, is of interest for the prediction of partial discharge inception and breakdown fields. Models for first electron production, streamer inception and leader propagation are presented, which are combined systematically to deduce these fields in technically relevant electrical insulation systems.