Gives a brief survey of aspects of research into air infiltration. Discusses methods of measurement of ventilation rates and of building leakiness, covering the infrasonic and pressurisation methods of leakage testing, and thermography and tracer gas methods of measuring ventilation rates. Describes the available techniques for predicting leakiness and infiltration rates, and discusses their ease of application and accuracy. Concludes that the achievements of research to date are sound methods of measuring leakiness and infiltration rates. Suggests that a requirement for more refined prediction is a programme of testing of leakage properties to derive mean values for different building types, which could be used as data for computer calculations of infiltration rates under different conditions. An appendix on permeable buildings is included.