a fuel injector , in particular a fuel injector for fuel - injection systems of internal combustion engines , including a piezoelectric or magnetostrictive actuator , which , via a valve needle , actuates a valve - closure member arranged on the valve needle , the valve - closure member cooperating with a valve - seat surface to form a sealing seat , the fuel injector having an hydraulic compensation chamber . a pressure piston cooperates with the compensation chamber , which is filled with hydraulic fluid via an hydraulic fluid inlet . the actuator is arranged between the pressure piston and the valve needle and displaceable in the axis of the valve needle and the pressure piston .