In the USA, full scale computer applications for HVAC related problems started in the early 1960s when the author was involved in the US government's projects to evaluate the thermal environment in fallout shelters by an hour by hour simulation of heat and moisture transfer processes between human occupants and the shelter's interior surfaces under a limited ventilation condition. General building energy simulations based on hour by hour calculations were started a few years later by the gas and electric industries. This led to the formation of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Task Group on Energy Requirements to develop a comprehensive hourly energy performance simulation of buildings, as well as to the activities of automated procedure for engineering consultants (APEC) for cooling load calculations. These activities eventually developed into the formation of four international symposia (Gaithersburg, Banff, Paris, and Tokyo) on the use of computers for environmental engineering related to buildings, the forerunner of IBPSA. A considerable amount of effort went into the earlier thermal simulation programs to improve the physical and empirical modeling of air, moisture and heat transfer processes in and through a complex building structure under varying weather conditions and building use conditions. Parallel and equally comprehensive efforts were made to improve the simulations of HVAC systems and equipment, and the development of typical weather data. (C) 2001 Elsevier Science B.V. All rights reserved.