It is particularly important to be aware of the air flow pattern in a building when determining indoor air quality problems or calculating space conditioning loads for energy consumption. Correct sizing of space conditioning equipment is also dependent upon accurate air flow information. A number of infiltration models have been developed to calculate infiltration-related energy losses and the resulting air flow distribution in both single-zone and multizone buildings. International infiltration research has been conducted since the early twenties - infiltration modelling, however, is a relatively new task. Most of the modelling effort has taken place during the last 15 years, during part of which time the Air Infiltration and Ventilation Centre has been in operation. This paper gives an overview of the development of infiltration models.