Carbon dioxide (CO₂)-based demand controlled ventilation (DCV) is increasingly used to modulate outside air ventilation based on real-time occupancy. Its use could potentially become as common as thermostatic control is today. This article summarizes the current state of the art in CO₂-based ventilation control including a brief discussion of the technology used, its reliability and how it is best applied. Like any control approach, the success of a CO₂-based DCV application is dependent on how it is engineered and installed.