The physiology of heat regulation and the science of clothing have both made significant advances since L. H. Newburgh (1) published his classic compendium of papers in 1949. During the past two decades, our better understanding of the regulation of body temperature during both rest and exercise has made possible the development of mathematiccll models (2, 3, 4) that predict quantitatively the principal temperature and energy exchange factors from the internal body to the skin surface for any chosen environmental condition. The skin surface serves as the boundary between man and his environment. The temperature and wettedness of the skin govern man•s heat exchange with the environment, which in turn can always be described (5} by an operative temperature (T0 ) and an ambient vapor pressure (Pa); the effective heat transfer coefficients involved ate functions of man•s clothing insulation, the ambient air movement, and 'barometri-c pressure.