This review of the science of building enclosures follows the developments in air control. Initially, building papers were thought to be sufficiently suitable. Now we recognize that the performance of the air barrier must be considered in light of its performance as a system. Assuming that air barriers are employed mainly for the sake of energy conservation is a mistake. Air transport control has been recognized as critical to the proper functioning of buildings and building enclosures. Airflow is related to all facets of environmental control because it affects the transport of heat and moisture and affects the indoor environment as well as the durability of the building enclosure. While the need for airtightness is now well recognized, achieving it in practice is a challenge.