A method is disclosed for monitoring the state of a piezoelectric injector of the fuel injection system of an internal combustion engine, the piezoelectric injector having a piezoelectric actuator and a nozzle needle that can be moved by said piezoelectric actuator. The piezoelectric injector can be operated in a partial-stroke mode and a full-stroke mode. In the partial-stroke mode, the curve of an electrical parameter over time is recorded, the maximum value of the curve is determined, a constant parameter value that arises after the presence of the maximum value is determined, the duration between the presence of the maximum value and the reaching of the constant parameter value is determined, the difference between the maximum value and the constant parameter value is determined, and conclusions about the state of the piezoelectric injector are drawn on the basis of the determined duration and the determined difference.