Abstract: An apparatus and method for monitoring a card slot is provided. The apparatus is for monitoring a card slot for the presence of unauthorised objects or devices and comprises a photon coupled interrupter having a light source configured for placement at a first side of the card slot to be monitored, the light source being configured to emit light across the card slot; a light sensor configured for placement at a side of the card slot opposite to the first side, the light sensor being configured to sense light from the light source that has propagated across the card slot; and control circuitry for controlling the light source to emit light and for determining the presence of an object or device in the card slot from a signal output from the light sensor.