The growing demand for air-conditioned buildings and the resultant demand for electrical energy has prompted research into passive cooling, such as the European Union funded PASCOOL programme. One of the tasks in this programme is to develop comfort criteria appropriate to free-running non air-conditioned buildings, where environmental conditions are likely to vary much more widely than mechanically controlled buildings. Conventional comfort theory has criteria which are more appropriate to controlled buildings. These criteria fail to account for the adaptive behaviour of the occupants to improve the thermal conditions actually experienced. This paper briefly summarises the results of the comfort monitoring surveys, conducted within the research programme during 1993 and 1994, which have provided information on room and local thermal conditions, and simultaneous subjective responses. Although limited in scope, the results show clearly that the subjects were exercising a considerable amount of adaptation both in regard to their person and their immediate surroundings during the surveys. More discursive comment is made on adaptive behaviour such as metabolic rate adjustment, followed by speculative comments on the nature of environmental tolerance. Finally, an outline for an expert system to assess thermal satisfaction is proposed.