Ventilation is the exchange of air between inside and outside the building and is a fundamental requirement resulting from occupancy. It is as important as the provision of daylight, and historically it has had almost as much influence on the development of plan, section and envelope. In contemporary buildings in temperate climates, natural ventilation is usually provided by openable windows, rather than specialised elements. This has not always been so, as typified by the Victorian fascination with ventilation, and the expression of these functional elements in schools, agricultural and factory buildings (Figure 7.1).