A monitoring circuit 14, 16, 18, 20, 22 for monitoring an operating parameter of an integrated circuit 2 comprises a ring oscillator circuit 80 comprising a plurality of serially connected inverting stages 82-1, 82-2, 82-3. At least one of the inverting stages 82-1, 82-2 comprises at least one leakage transistor 64-1, 64-2 which is configured to operate in a leakage mode in which substantially all current through the at least one leakage transistor is a leakage current, and a capacitive element 70-1 arranged to be charged or discharged in dependence on the leakage current. The ring oscillator circuit 80 thus generates an oscillating signal with an oscillation period dependent on a rate at which the capacitive element 70-1 is charged or discharged. The operating parameter controls a magnitude of the leakage current so that the oscillation period indicates the operating parameter.