An electrosurgical instrument for delivering microwave energy into biological tissue, the electrosurgical instrument comprising: a coaxial cable for conveying microwave energy, the coaxial cable having an inner conductor, an outer conductor, and a first dielectric material separating the inner conductor and outer conductor; a radiating tip portion disposed at a distal end of the coaxial cable to receive the microwave energy from the coaxial cable; and an imaging element for conveying an imaging signal to permit visualisation of a distal end of the instrument, wherein the radiating tip portion comprises: a dielectric tip formed from a second dielectric material that is different from the first dielectric material, and a distal conductive portion of the inner conductor, which extends longitudinally into the dielectric tip, wherein the second dielectric material has a dielectric constant greater than the first dielectric material, the geometry of the second dielectric material and its dielectric constant being selected such that an axial length of the dielectric tip corresponds to a fraction of a wavelength of the microwave energy when propagating in the dielectric tip, whereby the radiating tip portion is arranged to act as a quarter wave impedance transformer to match an input impedance to a tissue load impedance to radiate a localized microwave field for tissue ablation, wherein the imaging element extends through the first dielectric material and radiating tip portion within the inner conductor and distal conductive portion. a coaxial cable for conveying microwave energy, the coaxial cable having an inner conductor, an outer conductor, and a first dielectric material separating the inner conductor and outer conductor; a radiating tip portion disposed at a distal end of the coaxial cable to receive the microwave energy from the coaxial cable; and an imaging element for conveying an imaging signal to permit visualisation of a distal end of the instrument, wherein the radiating tip portion comprises: a dielectric tip formed from a second dielectric material that is different from the first dielectric material, and a distal conductive portion of the inner conductor, which extends longitudinally into the dielectric tip, a dielectric tip formed from a second dielectric material that is different from the first dielectric material, and a distal conductive portion of the inner conductor, which extends longitudinally into the dielectric tip, wherein the second dielectric material has a dielectric constant greater than the first dielectric material, the geometry of the second dielectric material and its dielectric constant being selected such that an axial length of the dielectric tip corresponds to a fraction of a wavelength of the microwave energy when propagating in the dielectric tip, whereby the radiating tip portion is arranged to act as a quarter wave impedance transformer to match an input impedance to a tissue load impedance to radiate a localized microwave field for tissue ablation, wherein the imaging element extends through the first dielectric material and radiating tip portion within the inner conductor and distal conductive portion. <EOS>