An electrostatic discharge protection circuit comprises at least two electrostatic discharge protection units connected in series between respective pairs of at least three input terminals, one of the input terminals being a reference input terminal. Each of the units comprises a silicon controlled rectifier and a current mirror. The output of the silicon controlled rectifier constitutes a first output of the respective unit and is connected to an input terminal of the circuit. The output of the current mirror constitutes a second output of the respective unit and is connected with the reference input terminal of the circuit. Thus the units are connected in series but the output terminals of the current mirrors are all connected with the reference input terminal, which may be a ground terminal, so as to minimize the breakdown resistance of the circuit.