A method of detecting and monitoring torsional oscillation in a mechanical coupling between a prime mover and a multiphase alternating current (AC) electrical generator, comprises the steps of: detecting in at least one received signal variations that are indicative of changes in the rotational speed of the electrical generator; extracting information from the detected variations that are indicative of torsional oscillation to produce a torsional oscillation signal that has information relating to at least the amplitude of the torsional oscillation; and monitoring the torsional oscillation signal to determine the presence and severity of the torsional oscillation in the mechanical coupling.