A portable electronic transaction device and a terminal therefor. The device comprises a central data processor, a fixed memory, an adaptable memory, optical means for transmitting and receiving information to and from the terminal and self-contained power source means, such as an on-board battery. The device is arranged to operate in either a stand-alone mode during which it monitors itself for abnormal conditions or in an operating mode during which it communicates with said terminal for the transmission of information therebetween. The device also includes cryptographic means for encrypting outgoing information and decrypting incoming information in accordance with cryptographic information stored in its memory. The cryptographic information is modified as a function of time by clock means in the device and/or a function of information transmitted to the device by the terminal or stored in the device. During the standby mode of operation the device monitors itself, whereas in its operational mode it communicates with the terminal. Power for the standy mode is provided, via the on-board battery. Power for the operation of the device in the operational mode is provided via photocells in the device which are arranged to receive light from lamp means in the terminal.