Natural ventilation reduces energy consumption for fans and mechanical cooling and in mostcases gives occupants control over their office space. Further benefits include no fan noise and insome cases elimination of the mechanical cooling system. The information in this paper has beenpresented to help building designers, owners and managers understand how certain key factorsaffect the performance and energy efficiency of the ventilation system, and to operate ventilationsystems at minimal energy cost. Improving the air tightness of older buildings will reduce airleakage and cold drafts, and help reduce energy use by improving the performance of ventilationsystems. There are three strategies for achieving acceptable indoor air quality: ventilation, sourcecontrol and cleaning/filtration. Depending on the building and the specific characteristics of itslocation, these strategies may be used singly or in combination. When identifiable contaminationsources are present in a building, it is necessary to reduce contamination sources as much aspossible either by using environmentally friendly furnishings, materials and products, or byexhausting contaminants at the source, if possible. General Ventilation should then be used. Forbuildings where the number of occupants varies significantly with time, such as office complexesand schools, it may be possible to further improve energy efficiency by turning off ventilationsystems during the non-occupied periods and controlling their ventilation rates during occupiedperiods based on the actual number of occupants at a given time.