The design and construction industry is increasingly interested in the effects of air leakage and how it relates to mechanical system efficiency, thermal comfort issues for building occupants, condensation, moisture accumulation, and concealed deterioration of building materials within wall and roof assemblies. Recent changes to the international building codes and ASHRAE Standard 90.1, Energy Standard for Building Except Low-Rise Residential Buildings, recognize and limit uncontrolled whole-building air leakage rates. This paper includes three case studies that highlight the importance of limiting uncontrolled air leakage through building enclosures by demonstrating the detrimental effects of air leakage as well as the benefits of improving airtightness in the same buildings. The first two case studies, which include a historic school building and a contemporary natatorium, showed that significant reductions in air leakage rates improved occupant comfort and eliminated condensation and moisture accumulation within wall and roof assemblies, respectively. The third case study, an existing 86,400-square-foot office building with an upgraded building enclosure, illustrated the impact of varying building enclosure air leakage rates on energy consumption using a calibrated whole-building energy model simulation. This energy model incorporated measured whole-building air leakage rates and was calibrated using one year of site-specific weather and submetered energy consumption of electric and mechanical systems.