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In general, malignant cells are more sensitive to heat than are normal cells in the range of 41-45°C. In addition, most clinically apparent tumors (above 1-cm diameter) have blood perfusion rates less than 1/5 that of surrounding normal tissue, meaning that they may be preferentially heated. Hyperthermic treatment may be local (tumor only), regional (e.g., a limb), or whole body. Physical techniques for hyperthermia include metabolic heat containment, conduction through the skin (e.g., hot water bath), perfusion of externally heated blood, heated intravenous fluids and anesthetic gases, ultrasound, and electro-magnetic EM coupling modalities. EM modalities include capacitive, inductive, and UHF-microwave radiative techniques, and may be invasive or noninvasive. Hyperthermia is effective against malignant cells not successfully attacked by ionizing radiation and also shows synergism with X-irradiation and some forms of chemotherapy making combination therapy attractive. Thermometric requirements vary with the treatment modality and clinical situation. Characteristics of tumors which may influence the choice of treatment are discussed. Local heating of deep-seated tumors with appropriate thermometry remains a technical challenge. Thermal dose requirements for various tumors and optimal protocols for adjuvant therapy are biological challenges.