This paper deals with the influence of electrical discharge machining (EDM) on surface and sub-surface quality in the manufacturing of mould and tool steel. The thermal nature of material removal by EDM yields a thermally affected zone at the surface of the manufactured part. This zone consists of a molten and resolidified layer, and a heat affected zone, showing properties that differ considerably from the base material. Based on experimental investigations with three types of EDM processes (sinking EDM, wire EDM and milling EDM), the influence of process parameters on surface and sub-surface properties is discussed. These include surface roughness, sub-surface micro-structure and composition, micro-hardness and residual stresses. Attention goes to the dangers of surface degradation, yet also to the opportunities to use the EDM process for surface improvement.