"A principal purpose of HVAC is to provide conditions for human thermal comfort@ ""that condition of mind that expresses satisfaction with the thermal environment"" (ASHRAE Standard 55). This definition leaves open what is meant by ""condition of mind"" or ""satisfaction@"" but it correctly emphasizes that judgment of comfort is a cognitive process involving many inputs influenced by physical@ physiological@ psychological@ and other processes. This chapter summarizes the fundamentals of human thermoregulation and comfort in terms useful to the engineer for operating systems and designing for the comfort and health of building occupants."