Building decarbonization is considered the cornerstone of global climate action, but success hinges on efficient systems design and precise performance verification through test and balance (TAB). This session explores the critical role of TAB in optimizing HVAC system performance in conjunction with commissioning to support energy efficiency, occupant wellness, and indoor environmental quality. Common TAB challenges in decarbonized and high-performance building challenges including system calibration, airflow imbalances, and sensor integration, and how these issues can compromise sustainability and comfort goals, will be addressed. Real-world examples will be used to demonstrate how advanced TAB methodologies, continuous commissioning, and data-driven diagnostics can enhance system functionality, reduce emissions, and align with the goals of LEED, WELL, and other green building certifications. The session will equip participants with actionable strategies to ensure that decarbonization efforts translate into measurable performance outcomes.