An electrosurgical instrument configured for delivering microwave frequency energy having a predetermined operating frequency into tissue having a predetermined characteristic impedance in contact with a distal end of the instrument, the instrument comprising: a proximal coaxial transmission line for conveying microwave frequency energy comprising a first inner conductor, a first outer conductor formed coaxially with the first inner conductor, and a first dielectric layer separating the first inner conductor and the first outer conductor; a distal coaxial transmission line for conveying microwave frequency energy comprising a second inner conductor connected to the first inner conductor, a second outer conductor formed coaxially with the second inner conductor and connected to the first outer conductor, and a second dielectric layer separating the second inner conductor and the second outer conductor; wherein a ratio of an inner diameter of the second outer conductor to the outer diameter of the second inner conductor is such that a characteristic impedance of the distal coaxial transmission line is intermediate between a characteristic impedance of the proximal coaxial transmission line and a load impedance at the distal end of the distal coaxial transmission line when the distal end of the instrument is in contact with the tissue; wherein a length of the distal coaxial transmission line is such that the distal coaxial transmission line is an impedance transformer that improves the impedance match between the proximal coaxial transmission line and the load impedance at the distal end of the distal coaxial transmission line when the distal end of instrument is in contact with the tissue, at the predetermined operating frequency; and characterised in that the electrosurgical instrument comprises a further distal coaxial transmission line comprising a third inner conductor connected to the second inner conductor, a third outer conductor formed coaxially with the third inner conductor and connected to the second outer conductor, and a third dielectric layer separating the third inner conductor and the third outer conductor; and wherein a length of the further distal coaxial transmission line is such that the further distal coaxial transmission line substantially cancels out a reactive part of the predetermined characteristic impedance of the tissue at the predetermined operating frequency. a proximal coaxial transmission line for conveying microwave frequency energy comprising a first inner conductor, a first outer conductor formed coaxially with the first inner conductor, and a first dielectric layer separating the first inner conductor and the first outer conductor; a distal coaxial transmission line for conveying microwave frequency energy comprising a second inner conductor connected to the first inner conductor, a second outer conductor formed coaxially with the second inner conductor and connected to the first outer conductor, and a second dielectric layer separating the second inner conductor and the second outer conductor; wherein a ratio of an inner diameter of the second outer conductor to the outer diameter of the second inner conductor is such that a characteristic impedance of the distal coaxial transmission line is intermediate between a characteristic impedance of the proximal coaxial transmission line and a load impedance at the distal end of the distal coaxial transmission line when the distal end of the instrument is in contact with the tissue; wherein a length of the distal coaxial transmission line is such that the distal coaxial transmission line is an impedance transformer that improves the impedance match between the proximal coaxial transmission line and the load impedance at the distal end of the distal coaxial transmission line when the distal end of instrument is in contact with the tissue, at the predetermined operating frequency; and characterised in that the electrosurgical instrument comprises a further distal coaxial transmission line comprising a third inner conductor connected to the second inner conductor, a third outer conductor formed coaxially with the third inner conductor and connected to the second outer conductor, and a third dielectric layer separating the third inner conductor and the third outer conductor; and wherein a length of the further distal coaxial transmission line is such that the further distal coaxial transmission line substantially cancels out a reactive part of the predetermined characteristic impedance of the tissue at the predetermined operating frequency.