Heat stress associated with wearing chemical protective clothing was discussed. Workers who are required to wear chemical protective clothing are exposed to conditions that could create a heat stress problem. For example, simply putting on a shirt and trousers results in a 40 percent reduction in heat loss by convection, radiation, and evaporation. Experimental studies have shown that in subjects who are wearing impermeable vapor barrier clothing while working, the body temperature and heart rate increase more rapidly than when impermeable clothing is not worn. The physiological heat balance was discussed. The role of metabolic heat, environmental heat load, conduction, convection, radiation, and evaporation in maintaining thermal equilibrium were considered. The concept of thermal burden and its relation to thermal equilibrium were discussed. Evaporative cooling and its relation to thermal equilibrium and the use of protective clothing were considered. Experimental studies have shown that protective clothing ensembles may create a microenvironment that causes an increase in physiological strain. Heat production by the body increases with increasing work rate leading to a performance decrement. The higher the environmental temperature, the greater the heat stress which also causes a performance decrement. Heat disorders were reviewed. Of the common heat disorders, heat stroke, heat exhaustion, and heat cramps, heat stroke is the most serious and life threatening consequence of heat stress. Human factors that are associated with heat stress, such as physical condition, level of acclimatization, degree of hydration, age, sex, and weight, were discussed. In order to optimize performance and protect against the effects of heat stress, workers should be physically fit, acclimatized, well hydrated, and maintain their ideal weight. The best chance for minimizing the effects of heat stress in workers wearing protective clothing is to establish an educational program that emphasizes recognizing heat stress symptoms and protective practices such as maintaining adequate hydration.