A security system for freight containers comprises: a locking device configured to reliably attach to the container and prevent unauthorized opening of the container doors; an electromechanical locking mechanism; and a GPS receiver so that unlocking may be authorized when the container is at a selected location. The system may further include a device to sense conditions affecting the container; and, a communication system to transmit the output of the sensing device to a system administrator located remotely. The locking and sensing devices may be capable of two-way communication with the system administrator. 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.