The scientific basis for the treatment of arthritis with locally applied heat or cold is reviewed. Experimental studies in vitro, in animals, in healthy subjects, and in patients are considered. Results of investigations of the effects of locally applied heat or cold on the deeper tissues of joints and on joint temperature in patients are not consistent. In general, locally applied heat increases and locally applied cold decreases the temperature of the skin, superficial and deeper tissues, and joint cavity. Most studies dealing with the effects of heat and cold on pain, joint stiffness, grip strength, and joint function in inflamed joints report beneficial effects. In vitro studies show that higher temperatures increase the breakdown of articular cartilage and tissues that contain collagen. Therefore, one goal of physical therapy should be to decrease intraarticular temperature in actively inflamed arthritic joints.