When a reverse voltage is applied a perfect diode does not conduct, but leaks a very tiny current in ľA or less, which is negligible in most circuits because it will be very much smaller, compared to current flowing in the forward direction. However, all diodes have a maximum reverse voltage (approx. 50V or more) and if this range is exceeded the diode will fail and pass a excess current in the reverse direction, called breakdown (e.g. zener diode).