We describe RFID tag embodiments that incorporate a nonlinear resonator that self-adapts to the driving frequency of a reader. More particularly we describe RF tag comprising: an adaptive resonator configured to receive energy from an electromagnetic field produced by an RF reader; an energy store to store energy received by said adaptive resonator; a transmitter and configured to communicate with an RF reader; and a switch to switch said tag between an energy receiving mode during which said adaptive resonator receives energy from said electromagnetic field and stores energy in said energy store; and a communication mode during which said transmitter communicates with an RF reader.
