A system using tags with embedded circuitry that can respond to interrogation by radio signals by returning identifying information unique to each tag, along with other data. Most tags are passive, in that they have no power source of their own, instead getting power from the electromagnetic signal used for their interrogation. Some tags have fixed data that they report, while others can be used to store and retrieve data. The technology has a growing number of applications that include inventory systems to track stock, identity card-based access control, contactless smartcards, and automated tollbooths.