The effects of heat and its possible significance in cancer therapy have been documented for more than a century. Hyperthermia has been widely investigated as an alternative or adjuvant to conventional therapies such as surgery, radiation, and chemotherapy. The clinical reports are interesting, notwithstanding some harmful side effects and other complications which, however, may be attributable to an advanced stage of the disease at the time of treatment. In view of the extreme difficulty in matching suitable controls for cancer patients undertaking such therapy, it is essential to extend knowledge of the effects of hyperthermia on normal and neoplastic tissue by experimental studies. Further, the different methods of utilizing heat therapy have need for substantial technological improvements. To stimulate interest in this area, we have gathered together in this review some of the more pertinent reports on the application of hyperthermia to neoplastic and normal tissues as observed clinically and in the laboratory.