Techniques are disclosed for detecting shoplifting or theft, particularly in a retail environment, using radio-frequency identification ("RFID"). Preferred embodiments leverage RFID tags on merchandise and RFID data that is written at the point of sale on the merchandise-borne RFID tags themselves, on the customer's receipt, or both. Some embodiments also leverage RFID tags on customer identification or "loyalty" cards. After writing RFID data on the merchandise-borne tags and/or receipt, a matching operation is performed at an RFID reader when the shopper and his merchandise exit the premises, in order to determine whether the shopper has paid for the items in his possession.