Whole building airtightness testing has been a code compliance option in the International Energy Conservation Code and ASHRAE 90.1 for some time. That compliance option was rarely used and only became mandatory as part of building owners' requirements or a specific state or jurisdictional requirement that was limited to one or two states. As we moved towards Net Zero, building codes “tightened” up, and it is expected this will become a test that is mandatory on a more countrywide basis. It seems that time has arrived, with many states now requiring mandatory whole building airtightness testing. As we move toward those requirements, it is important to understand the latest in test methods, current and projected code requirements, how to specify the test properly, coordination needed with the construction team, and what type of pre-qualifications exist that you can rely upon.