Increasing evidence of the beneficial effects of hyperthermia in the treatment of tumors, particularly when combined with radiation therapy or chemotherapy, has created the need for practical methods of localizing heat in arbitrary tissue volumes. Heating superficial transplanted animal tumors in hot-water baths has been of considerable importance in research, but the method has very limited clinical application. Microwave diathermy and ultrasound diathermy are capable of greater penetration than external conduction heating, but are also generally limited to the heating of superficial tissue volumes. A more direct method for localized tissue heating is proposed: localized electric current fields produced with carefully specified electrode configurations. This technique, although often quite invasive, offers the possibility of heat localization in almost any treatment volume specified by the therapist. Variations in electrical and thermal properties of tissues can be exploited to improve the architecture of the treatment plan. Several electrode configurations are illustrated. Limitations of the technique are discussed.