An apparatus and method for ablating tissue is disclosed. The apparatus comprises a source of microwave radiation (1), a probe (5) for directing the microwave radiation into tissue, one or more detectors for detecting the power and phase of the microwave radiation and an impedance adjuster (50) for adjusting impedance so as to minimize the amount of microwave radiation which reflected back through the probe. The detector or detectors use a local oscillator (230) to derive the phase information. A modulator for modulating the microwave radiation to a cutting frequency is also disclosed.