Numerical simulation is useful for both the study of the fundamental processes such as, e.g., the protein denaturation at high temperatures, intracellular and extracellular ice formation, ice propagation in the tissues etc. and solution of the practical problems—in both the hyperthermia and the cryosurgery the control of the intensity and location of the action is of the paramount importance. Due to the restrictions of the real-time temperature monitoring, the mathematical modelling is extremely important.