Local withdrawal of heat from the body by superficial cold or cryotherapy rapidly lowers the temperature of the skin and subcutaneous tissue, as well as causing delayed cooling in deeper tissues. Since skin and subcutaneous tissue are very poor heat conductors, a temperature reduction of 5 °C in a 2-cm deep muscle layer takes about 20 min. It is therefore distinguished between short-term, intermittent, and long-term applications. In order to retain the core body temperature, application of cold to the skin initially results in vasoconstriction, with consequent repeated reactive vasodilation thereafter. Short-term cryotherapy already increases the pain threshold; long-term cryotherapy of 20–30 min decreases acute inflammation, local metabolism, and muscle spindle and Golgi tendon organ firing rates; slows nerve conduction velocity, relaxes muscles, and increases stiffness of connective tissue and joints. Localized heating of the body by 40–45 °C increases circulation, soft tissue elasticity, nerve conduction velocity, and cell membrane permeability; causes muscle relaxation and decreases joint stiffness. Superficial heating treatments achieve their maximum tissue temperature in the skin and subcutaneous fat. However, short-wave, decimeter-wave, microwave, and ultrasound therapy can heat deep tissues directly. In summary, both heat and heat withdrawal help to decrease painful muscle spasms. For treatment of acute inflammation and neurogenic spasticity, or after musculoskeletal injuries or burns, cryotherapy is indicated. Therapeutic heat helps to decrease joint stiffness and enhance connective tissue elasticity.