Thermal stress, whether heat or cold, can increase the risk of accidents in the workplace by causing fatigue. The Occupational Safety and Health Administration (OSHA) do not have a special standard for thermal stress. Heat stress is of great concern in the workplace, as well as during recreational activities. It is caused by a combination of factors and tends to increase body temperature, heart rate, and sweating. There are four basic sources of heat to the body, which includes radiation, convection, conduction, and metabolic. Extended exposures to heat extremes bring about a wide variety of heat-induced disorders. This chapter discusses the heat-induced disorders such as: heat rash, heat cramps, heat syncope, dehydration, heat exhaustion, and heat stroke. As with any hazard associated with the workplace, OSHA requires that engineering controls, administrative controls, and personal protective equipment be used in that order to prevent work-related injuries.