An electrosurgical system has an electrosurgical generator with a multiple-phase RF output stage coupled to a multiple-electrode electrosurgical instrument. The instrument has three treatment electrodes each of which is coupled to a respective generator output driven from, for instance, a three-phase output transformer. Continuous RF output voltage waveforms are simultaneously delivered to respective generator outputs at the operating frequency, each waveform being phase-displaced with respect to the other waveforms. The magnitude of the RF output voltage waveform delivered to at least one of the generator outputs is sufficient to cause tissue vaporisation at the respective treatment electrodes when the system is used for tissue treatment. Also disclosed is an electrosurgical generator in combination with an instrument having both bipolar coagulation electrodes and a remote return pad, the generator including means for varying the RF voltage outputs to vary the relative preponderance of bipolar and monopolar coagulation.