Almost all historic buildings were ventilated naturally (open windows!) although this effect has been compromised by the addition of partition walls, closures and mechanical systems. With an increased awareness of the cost and environmental impacts of energy use, natural ventilation has become an increasingly attractive method for reducing energy. The cost for providing acceptable indoor environmental quality and maintaining a healthy, comfortable, and productive indoor climate rather than the more prevalent approach of using mechanical ventilation is an interesting proposition. Natural ventilation systems rely on pressure differences to move fresh air through buildings. Pressure differences can be caused by wind or the buoyancy effect created by temperature differences or differences in humidity. In either case, the amount of ventilation will depend critically on the size and placement of openings in the building. It is useful to think of a natural ventilation system as a circuit, with equal consideration given to supply and exhaust. Openings between rooms such as transom windows, louvers, grills, or open plans are techniques to complete the airflow circuit through a building. Code requirements regarding smoke and fire transfer present challenges to the designer of a natural ventilation system. For example, historic buildings used the stairway as the exhaust stack, a technique now prevented by fire regulations in many cases. Simple basic consideration of the ‘condition of the air’ -, i.e. temperature and humidity and prevailing external wind flows can be used to advantage. By monitoring these conditions, applying sensors, remotely actuated controls and an optimized design such as using the venturi effect and the mass effect of the building structure. This can reduce not only the energy required but create a more favorable work environment with enhanced sustainability. Natural ventilation may have been opening a window in the past but as it can save 10/15% of the energy requirements of a building it is now an important design feature.