An electrosurgical instrument comprising: a coaxial feed cable for conveying microwave energy and/or radiofrequency energy, the coaxial feed cable having an inner conductor, an outer conductor, and a dielectric material separating the inner conductor and the outer conductor; an instrument tip disposed at a distal end of the coaxial feed cable to receive the microwave energy and/or the radiofrequency energy; and a fluid channel for conveying fluid to the instrument tip; wherein the instrument tip comprises: a dielectric body; an energy delivery structure (218, 220) for delivering the microwave energy and/or the radiofrequency energy into biological tissue, the energy delivery structure being formed in and/or on the dielectric body; and a hollow needle in fluid communication with the fluid channel, the hollow needle being arranged to deliver fluid from the fluid channel to a treatment site, wherein the hollow needle is movable relative to the instrument tip between: a retracted position, in which a distal end of the hollow needle is set back from a distal end of instrument tip; and an exposed position, in which the distal end of the hollow needle protrudes beyond the distal end of the instrument tip; wherein the energy delivery structure comprises a radiating structure for radiating the microwave energy into biological tissue, and wherein the hollow needle is electrically connected to the outer conductor to ground the hollow needle. a coaxial feed cable for conveying microwave energy and/or radiofrequency energy, the coaxial feed cable having an inner conductor, an outer conductor, and a dielectric material separating the inner conductor and the outer conductor; an instrument tip disposed at a distal end of the coaxial feed cable to receive the microwave energy and/or the radiofrequency energy; and a fluid channel for conveying fluid to the instrument tip; wherein the instrument tip comprises: a dielectric body; an energy delivery structure (218, 220) for delivering the microwave energy and/or the radiofrequency energy into biological tissue, the energy delivery structure being formed in and/or on the dielectric body; and a hollow needle in fluid communication with the fluid channel, the hollow needle being arranged to deliver fluid from the fluid channel to a treatment site, a dielectric body; an energy delivery structure (218, 220) for delivering the microwave energy and/or the radiofrequency energy into biological tissue, the energy delivery structure being formed in and/or on the dielectric body; and a hollow needle in fluid communication with the fluid channel, the hollow needle being arranged to deliver fluid from the fluid channel to a treatment site, wherein the hollow needle is movable relative to the instrument tip between: a retracted position, in which a distal end of the hollow needle is set back from a distal end of instrument tip; and an exposed position, in which the distal end of the hollow needle protrudes beyond the distal end of the instrument tip; a retracted position, in which a distal end of the hollow needle is set back from a distal end of instrument tip; and an exposed position, in which the distal end of the hollow needle protrudes beyond the distal end of the instrument tip; wherein the energy delivery structure comprises a radiating structure for radiating the microwave energy into biological tissue, and wherein the hollow needle is electrically connected to the outer conductor to ground the hollow needle.