Abstract: An electrosurgical apparatus and method for performing thermal treatment in the gastrointestinal tract, e.g. to ablate duodenal mucosal tissue. The apparatus comprises an instrument having a flexible cable and an applicator suitable for use with a gastroscope, which can be deployed within a patient to delivery energy in a targeted or otherwise controllable manner. The applicator can deliver microwave energy by radiation. The direct and depth-limited nature of microwave energy can be make it more effective than treatments that rely on thermal conduction. The applicator may include a radially extendable portion arranged to move an microwave energy delivery structure into contact with duodenal mucosal tissue at the treatment region. The applicator may comprise any of a balloon, bipolar radiator, movable paddle, and rotatable roller element.