A container security system has a container interior monitor portion with a container interior light sensing portion responsive to visible light. A different embodiment involves a monitoring device with a support, a door engaging member movable to and from an operational position, and a detection portion that generates an electrical signal in response to movement of the member away from the operational position. The support may be configured to be supported on an edge portion of a movable door. Alternatively, the monitoring device may include a wireless communication portion, and circuitry responsive to the signal and operatively coupled to the wireless communication portion.