The conventional theory of thermal comfort in conditions characteristic for dwellings and offices (for example, that of Fanger) assumes steady-state conditions. Yet thermal conditions in buildings are seldom steady, due to the interaction between building structure, climate, occupancy, and HVAC system. This article reviews work on thermal comfort specifically undertaken to examine what variations in indoor temperatures may be acceptable. Following an account of man's thermoregulatory system, some experimental findings on periodic and on ramp (or drift) variation in room temperature are presented. It is concluded that the results for cyclic variations uphold the present ASHRAE standard, but those for drifts may not.