A mobile device for use in processing a transaction, comprises an electronic card, tag, fob or similar small portable device having a stored identity and co mprising a housing having vertical and horizontal dimensions of a standard credit card, a tag, a fob or the like, transmitter/receiver means including an RFID transmitter and configured to communicate with a local computer for receiving messages, processor means for receiving, storing and outputting messages, and an output device configured to reproduce messages in visible or audible form. The aforesaid card, tag, fob or similar small portable device may be used in association wit h apparatus for reading and processing the card, tag, fob or similar small portable device consequent on a transaction involving a user of said card, tag, fob or similar small portable device, said apparatus comprising a reader for electronic remote reading of the stored identity on a card, tag, fob or similar small portable device, a processing station for processing a transaction involving the holder of the card, tag, fob or similar small portable device, a local computer configured to receive the identity read by the reader and transaction information from the processing station, and to cause a message to be transmitted to the card, tag, fob or similar small portable device, and one or more remote computers configured to contain a database of identities and transactions for the cards, tags, fobs or similar small portable devices and to generate messages for storage on the cards, tags, fo bs or similar small portable devices, the local computer being configured to alert said remote computer or one of said remote computers on reading of a stored identity, and to supply information for use at said local computer concerning one or more messages to be transmitted to and stored on said card, tag, fob or similar small portable device.