Abstract: A security system for freight containers comprises: a locking device to reliably attach to the container and prevent unauthorized opening of the container doors; a sensing device to sense conditions affecting the container; and, a communication system configured to transmit the output of the sensing device to a system administrator located remotely from the container. The locking and sensing devices may be capable of two-way communication with the system administrator, whereby the administrator may interrogate the locking/sensing device at selected times. The locking device may contain a GPS receiver whereby its geographic location at various times may be monitored either continuously, periodically, or after a trip is completed. The system may include a visual display on the exterior of the container capable of displaying selected messages. The messages may be preprogrammed or may be changed as the container moves from one geographic location to another.