Recent studies have advanced our knowledge of the mechanisms of death from electric shock. It is known that biological effects are related to the passage of current rather than to applied voltage. The paper considers only the fatalities resulting from accidents at power frequencies. There are three possible mechanisms, of death: ventricular fibrillation (incordinate action of the heart), respiratory arrest (at one time believed to be very common, and hence the usual advice on artificial respiration) and asphyxia resulting from a prolonged ‘hold-on’ type of shock passing across the chest. Unfortunately the appearances at post mortem do not help to distinguish between these different mechanisms of death, and so reliance must be placed on circumstantial evidence, in which the pathway of the current through the body is important. It is probable that the great majority of electrical fatalities are due to ventricular fibrillation, which brings the circulation to a standstill. The currents and times necessary to produce this effect have been suggested; but in deriving these, certain assumptions have been made. These assumptions are discussed, for it is important to recognise them when assessing the use of devices such as earth-leakage circuit breakers.