The development of an effective on-line computer control of Electrical Discharge Machining (EDM) is discussed. As the debris size and concentration have been known to affect the gap condition and also process performance, an experimental and theoretical investigation into the formation and ejection of debris has been initiated. A theoretical expression for the particle size has been obtained in terms of pulse energy and the velocity of ejection. The debris particles at various machining conditions were collected and measured. A log-normal distribution has been found to adequately represent the data. Empirical relationships for erosion rate and surface roughness with debris size have been obtained. The theoretical estimates of the debris size have been compared with the experimental values.