Abstract: The invention concerns an apparatus for monitoring the operation of electrical circuits on a vehicle trailer 10, and has a monitoring unit 18 including a monitor circuit 32 for outputting a signal in response to failure of a component such as a lamp 12a-12d of a trailer electrical circuit. The monitoring unit also has a transmitter 34 for passing a failure indication signal to a receiver 90 of an indicator unit 26 which is mounted in a towing vehicle 14 in a position where it can give an indication of failure of the component to the vehicle driver.