At first they were little more than a wireless version of a barcode, mostly used for stocktaking. But from these mundane beginnings radio-frequency identification (RFID) tags are spreading, and they could soon be keeping tabs on every one of us. This has privacy advocates worried, and they are campaigning for safeguards to be built into the still-to-be-finalised standards for RFID. Most RFID tags are nothing more than a simple combination of a chip and antenna, and act as a passive transponder. When prompted by a signal from a reading device, the tag sends back a digital code stored in its memory. The power the tag needs comes from the reader's radio signal.
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