||Booster fans, large underground fans, can increase the volumetric efficiency of ventilation systems by helping to balance the pressure and quantity distribution throughout a mine, reducing leakage and reducing the total power requirement. The increased volumetric efficiency and reduced system power consumption in ventilation systems utilizing booster fans is demonstrated using a VnetPC ventilation model of an underground room and pillar coal mine. In this mine, a booster fan has the potential to reduce the total flow volume by 58 m3/s and to reduce the air power requirement by 110 kW. Though booster fans have the potential to increase the efficiency of coal mine ventilation systems, there are problems associated with their use, most notably recirculation, the leakage of contaminated return air into the intake air. Recirculation is a risk because it has the potential to increase the concentration of air contaminants, including methane, dust, and heat, in the intake air. Computational Fluid Dynamic (CFD) and Ventsim numerical models and a physical laboratory model are used to evaluate recirculation in ventilation systems with booster fans. The use of booster fans does increase the potential for recirculation in coal mine ventilation systems, but the potential for recirculation is strongly dependent on the size of the booster fan relative to the main surface fan, the location of the booster fan, and the quality of the ventilation control devices. The potential for recirculation increases as the booster fan pressure increases relative to the main fan, as the booster fan is located closer to the development sections, and as the quality of the ventilation control devices decreases. In well-designed and well-maintained ventilation systems, recirculation can be managed. In part because of the risk of recirculation, booster fans are prohibited in most underground coal mines in the United States. However, despite increased potential for recirculation, booster fans are used effectively in other major coal producing countries including Australia and the United Kingdom. Regulations specific to coal mine ventilation from Australia, the United Kingdom, and the United States are compared to help identify practices to reduce risks associated with the use of booster fans.