The underlying principle behind new minimal invasive procedures, such as microwave thermotherapy, is to coagulate the prostatic adenomatous tissue by means of heat. This article describes the action of heat on tissue and identifies areas of concern during treatment. The extent of the necrosis during treatment is governed by two physical variables: the intraprostatic temperature and the duration of the heat exposure. The prostatic blood flow is a key factor for the outcome of microwave treatment because it acts as a coolant and may effectively sink the temperature in the treatment area. Blood flow can vary substantially between patients and may change significantly during treatment. By measuring the intraprostatic temperature and varying the microwave power accordingly, it is possible to compensate for the large variations in prostatic blood flow and obtain consistent treatment.