Electric Discharge Machining process is investigated both theoretically and experimentally to determine the effects of electrode materials on the machining performance. For this purpose a single and isolated spark is physically and mathematically modelled, and its three phases; viz., Breakdown, Discharge and Erosion are investigated. Resolidified electrode materials as suspended particles in the dielectric liquid are found to be the most significant factor in the breakdown phase. Mathematical expressions relating the time lags to particle concentration are given which can be used to determine the effects of particle concentration on the machining performance. Discharge properties are shown to be dependent on the discharge medium which includes vapours of the electrode materials. The polarity effect has been studied both theoretically and experimentally. Some qualitative explanation is given for the erosion phase. Importance of electrical forces is discussed and a simple mathematical expression is given for the erosion phase. It is concluded that optimum machining conditions can only be obtained by proper selection of the tool material, workpiece material and discharge medium since they affect the initiation and development of the discharge and erosion of electrode materials.