Air distribution systems bring conditioned (heated and cooled) air to people occupying a building, and therefore directly affect occupant comfort. Over the last several decades, significant improvements have been made to the design of air distribution systems as well as to the way in which these systems are controlled. These improved designs and controls can result in dramatic energy savings, yet many buildings continue to rely on obsolete, inefficient systems for this critical function. The energy savings achieved in the Retrocommissioning, Lighting, and Supplemental Load Reductions stages are likely to have reduced the load on the building’s HVAC system, sometimes considerably. But before evaluating the potential to replace the existing heating and/or cooling equipment with smaller and more-efficient equipment, optimize the efficiency of the air distribution system itself. Doing so may well enable even greater savings and a reduction in required heating and cooling equipment capacity.