An electrosurgical instrument which is capable of simultaneously ablating an area of tissue with microwave energy and performing resection with RF energy. The instrument comprises a structure for conveying both RF and microwave energy to an instrument tip that is configured to emit the microwave energy in a manner suitable for ablation (e.g. as a substantially spherical field) and to emit the RF energy in a more focussed manner to enable accurate and controllable resection to be performed. The energy conveying structure comprises a coaxial transmission line for conveying microwave energy. The coaxial transmission line has a hollow inner conductor that defines a passage that supports a second transmission line for conveying radiofrequency energy.