Privacy advocates have fretted for some time about radio-frequency identification (RFID) tags and their potential for enabling detailed tracking of an individual’s activities. Some tags are obvious, such as those used for E-Z Pass toll collection on northeastern U.S. highways; others are more insidious, such as those tucked inside U.S. passports and on the London Underground’s travel cards. Rapidly expanding in use, such tags may soon even report the amount of material in recycle cans.