Ventilation is the intentional introduction of outdoor air into a space and is mainly used to control indoor air quality by diluting and displacing indoor pollutants; it can also be used for purposes of thermal comfort or dehumidification.

The intentional introduction of outdoor air can be categorized as either mechanical ventilation, or natural ventilation. Mechanical ventilation uses fans to drive the flow of outdoor air into a building. This may be accomplished by pressurization (in the case of positively pressurized buildings), or by depressurization (in the case of exhaust ventilation systems). Many mechanically ventilated buildings use a combination of both, with the ventilation being integrated into the HVAC system. Natural ventilation is the intentional passive flow of outdoor air into a building through planned openings (such as louvers, doors, and windows). Natural ventilation does not require mechanical systems to move outdoor air, it relies entirely on passive physical phenomena, such as diffusion, wind pressure, or the stack effect. Mixed mode ventilation systems use both mechanical and natural processes. The mechanical and natural components may be used in conjunction with each other or separately at different times of day or season of the year. Since the natural component can be affected by unpredictable environmental c onditions it may not always provide an appropriate amount of ventilation. In this case, mechanical systems may be used to supplement or to regulate the naturally driven flow.

In many instances, ventilation for indoor air quality is simultaneously beneficial for the control of thermal comfort. At these times, it can be useful to increase the rate of ventilation beyond the minimum required for indoor air quality. Two examples include air-side economizer strategies and ventilative pre-cooling. In other instances, ventilation for indoor air quality contributes to the need for - and energy use by - mechanical heating and cooling equipment. In hot and humid climates, dehumidification of ventilation air can be a particularly energy intensive process.

Ventilation should be considered for its relationship to "venting" for appliances and combustion equipment such as water heaters, furnaces, boilers, and wood stoves. Most importantly, the design of building ventilation must be careful to avoid the backdraft of combustion products from "naturally vented" appliances into the occupied space. This issue is of greater importance in new buildings with more air tight envelopes. To avoid the hazard, many modern combustion appliances utilize "direct venting" which draws combustion air directly from outdoors, instead of from the indoor environment.

Natural ventilation can also be achieved through the use of operable windows, this has largely been removed from most current architecture buildings due to the mechanical system continuously operating. The United States current strategy for ventilating buildings is to rely solely on mechanical ventilation. In Europe designers have experimented with design solutions that will allow for natural ventilation with minimal mechanical interference. These techniques include: building layout, facade construction, and materials used for inside finishes. European designers have also switched back to the use of operable windows to solve indoor air quality issues. "In the United States, the elimination of operable windows is one of the greatest losses in contemporary architecture.

refers to any system that uses mechanical means, such as a fan, to introduce subaerial air to a space. This includes positive pressure ventilation, exhaust ventilation, and balanced systems that use both supply and exhaust ventilation.

refers to intentionally designed passive methods of introducing subaerial to a space without the use of mechanical systems.

systems use both natural and mechanical processes.

is the uncontrolled flow of air from outdoors to indoors through leaks (unplanned openings) in a building envelope. When a building design relies on environmentally driven circumstantial infiltration to maintain indoor air quality, this flow has been referred to as adventitious ventilation.