Existing buildings often have hidden operational issues that increase energy costs, strain maintenance teams, reduce equipment reliability, and impact occupant comfort. Retro-commissioning provides facility owners with a practical, data-driven process to identify and correct these issues before pursuing major capital upgrades. This presentation will provide an overview of retro-commissioning, including when it should be considered, how the process is typically performed, what types of building systems are evaluated, and how data and field verification are used to confirm results. Attendees will learn how retro-commissioning can help improve HVAC, controls, ventilation, lighting, and overall system performance, while supporting long-term financial, operational, and sustainability goals across a variety of facility types.