An electrosurgical apparatus for treating biological tissue with microwave energy, the apparatus comprising: a microwave energy signal generator for generating a microwave energy waveform; an electrosurgical instrument arranged to deliver the microwave energy waveform from a distal end thereof for tissue treatment; a controller in communication with the microwave energy signal generator the microwave energy signal generator being configured to deliver the microwave energy waveform as one microwave energy signal pulse, and the controller being configured to control the profile of the one microwave energy signal pulse to cause ablation or coagulation of the biological tissue and to substantially prevent the one pulse from causing heat to build-up in the electrosurgical instrument, wherein the controller is configured to control the profile of the one pulse such that a peak power of the one pulse is maintained at or above a peak power minimum which is set to cause ablation or coagulation of the biological tissue during the one microwave energy signal pulse, the peak power minimum being 500 W, and wherein at least one of the following applies: (a) the controller is configured to control the profile of the one pulse such that a duration of an ON portion of the one pulse is maintained at or below an ON portion duration limit which is set to substantially prevent the microwave energy waveform from causing dielectric heating of the electrosurgical instrument during the one pulse, the ON portion duration limit being 1 s; (b) the controller is configured to control the profile of the one pulse such that a duty cycle of the one pulse is maintained at or below a duty cycle limit which is set such that heat which the microwave energy waveform causes to be built up in the electrosurgical instrument during an ON portion of the one pulse substantially dissipates during an OFF portion of the one pulse. a microwave energy signal generator for generating a microwave energy waveform; an electrosurgical instrument arranged to deliver the microwave energy waveform from a distal end thereof for tissue treatment; a controller in communication with the microwave energy signal generator the microwave energy signal generator being configured to deliver the microwave energy waveform as one microwave energy signal pulse, and the controller being configured to control the profile of the one microwave energy signal pulse to cause ablation or coagulation of the biological tissue and to substantially prevent the one pulse from causing heat to build-up in the electrosurgical instrument, wherein the controller is configured to control the profile of the one pulse such that a peak power of the one pulse is maintained at or above a peak power minimum which is set to cause ablation or coagulation of the biological tissue during the one microwave energy signal pulse, the peak power minimum being 500 W, and wherein at least one of the following applies: (a) the controller is configured to control the profile of the one pulse such that a duration of an ON portion of the one pulse is maintained at or below an ON portion duration limit which is set to substantially prevent the microwave energy waveform from causing dielectric heating of the electrosurgical instrument during the one pulse, the ON portion duration limit being 1 s; (b) the controller is configured to control the profile of the one pulse such that a duty cycle of the one pulse is maintained at or below a duty cycle limit which is set such that heat which the microwave energy waveform causes to be built up in the electrosurgical instrument during an ON portion of the one pulse substantially dissipates during an OFF portion of the one pulse. (a) the controller is configured to control the profile of the one pulse such that a duration of an ON portion of the one pulse is maintained at or below an ON portion duration limit which is set to substantially prevent the microwave energy waveform from causing dielectric heating of the electrosurgical instrument during the one pulse, the ON portion duration limit being 1 s; (b) the controller is configured to control the profile of the one pulse such that a duty cycle of the one pulse is maintained at or below a duty cycle limit which is set such that heat which the microwave energy waveform causes to be built up in the electrosurgical instrument during an ON portion of the one pulse substantially dissipates during an OFF portion of the one pulse.