An electrosurgical apparatus for treating biological tissue, the apparatus comprising: an electrosurgical generator arranged to supply microwave electromagnetic energy; a tissue-freezing fluid supply; an electrosurgical instrument connected to receiving the microwave EM energy from the electrosurgical generator and to receive the tissue-freezing fluid from the tissue-freezing fluid supply; and a controller configured to: cause the tissue-freezing fluid to flow through a fluid feed to a thermal transfer portion to freeze biological tissue in the treatment zone; detect conditions in the treatment zone to determine if biological tissue in the treatment zone is frozen; and in response to determining that biological tissue in the treatment zone is frozen, cause the microwave energy to be delivered from the radiating tip, wherein the electrosurgical instrument for treating biological tissue comprises: a transmission line for conveying microwave electromagnetic energy; a radiating tip mounted at a distal end of the transmission line to receive and radiate the microwave EM energy from the transmission line into a treatment zone around the radiating tip; the fluid feed for conveying a tissue-freezing fluid to the treatment zone; and the thermal transfer portion connected to receive the tissue-freezing fluid from the fluid feed at a distal end of the transmission line, wherein the thermal transfer portion is arranged to provide thermal communication between the tissue-freezing fluid and biological tissue in the treatment zone to freeze the biological tissue in the treatment zone, and wherein the thermal transfer portion includes a tissue-freezing element that is movable between an exposed position where it protrudes distally beyond the radiating tip, and a retracted position in which it is set back from the radiating tip. an electrosurgical generator arranged to supply microwave electromagnetic energy; a tissue-freezing fluid supply; an electrosurgical instrument connected to receiving the microwave EM energy from the electrosurgical generator and to receive the tissue-freezing fluid from the tissue-freezing fluid supply; and a controller configured to: cause the tissue-freezing fluid to flow through a fluid feed to a thermal transfer portion to freeze biological tissue in the treatment zone; detect conditions in the treatment zone to determine if biological tissue in the treatment zone is frozen; and in response to determining that biological tissue in the treatment zone is frozen, cause the microwave energy to be delivered from the radiating tip, cause the tissue-freezing fluid to flow through a fluid feed to a thermal transfer portion to freeze biological tissue in the treatment zone; detect conditions in the treatment zone to determine if biological tissue in the treatment zone is frozen; and in response to determining that biological tissue in the treatment zone is frozen, cause the microwave energy to be delivered from the radiating tip, wherein the electrosurgical instrument for treating biological tissue comprises: a transmission line for conveying microwave electromagnetic energy; a radiating tip mounted at a distal end of the transmission line to receive and radiate the microwave EM energy from the transmission line into a treatment zone around the radiating tip; the fluid feed for conveying a tissue-freezing fluid to the treatment zone; and the thermal transfer portion connected to receive the tissue-freezing fluid from the fluid feed at a distal end of the transmission line, wherein the thermal transfer portion is arranged to provide thermal communication between the tissue-freezing fluid and biological tissue in the treatment zone to freeze the biological tissue in the treatment zone, and wherein the thermal transfer portion includes a tissue-freezing element that is movable between an exposed position where it protrudes distally beyond the radiating tip, and a retracted position in which it is set back from the radiating tip. a transmission line for conveying microwave electromagnetic energy; a radiating tip mounted at a distal end of the transmission line to receive and radiate the microwave EM energy from the transmission line into a treatment zone around the radiating tip; the fluid feed for conveying a tissue-freezing fluid to the treatment zone; and the thermal transfer portion connected to receive the tissue-freezing fluid from the fluid feed at a distal end of the transmission line, wherein the thermal transfer portion is arranged to provide thermal communication between the tissue-freezing fluid and biological tissue in the treatment zone to freeze the biological tissue in the treatment zone, and wherein the thermal transfer portion includes a tissue-freezing element that is movable between an exposed position where it protrudes distally beyond the radiating tip, and a retracted position in which it is set back from the radiating tip.