Object and document control system

An electronic tag for the identification and/or secure control of an object or document includes a microcircuit (15), means (13, 14) for communicating with an interrogator, and presence detection means such as a tuned circuit (16, 17). The presence detection means (16, 17) may be reversibly activated and deactivated, for example, via a programmable transistor (75) with a floating gate (77). The presence detection means (16, 17), when activated, can be operated at a lower interrogation signal power than is required for energising the microcircuit (15). The electronic tag may also have a second presence detecting means, such as a soft magnetic strip, which cannot be deactivated.