A control system, for a machine tool of the kind in which electrical energy passes across a gap between a tool electrode and a workpiece to erode the workpiece, comprises means for forming signals representative of the rate of erosion of the workpiece and the rate of undesirable erosion or "wear" of the tool electrode, and a computer means which receives the signals and combines them with a signal received from a data input device, which signal represents a desired optimum relationship between the two rates of erosion, and forms an appropriate output signal for controlling the machine tool. The data input device is usable, in rough machining operations where the tool electrode may be relatively cheaper than machine time, to ensure that machining is done in the shortest time by allowing a relatively high rate of tool wear and, in fine machining where the tool electrode is expensive, to ensure minimal tool wear at the cost of machine time.