This report discusses the thermal distribution in living tissue during hyperthermic treatment with visible or infrared electromagnetic irradiation. The underlying physical mechanisms that determine the transient as well as the steady-state temperature distribution are discussed. Results from the treatment of a murine tumor model, C3H/Tif mammary adenocarcinoma in C3D2F1/Bom mouse strain, are reported. Total cure, defined by a nonrecurrent tumor, was obtained in 30% of the cases.