Seattle has required whole-building air barrier testing for nearly all commercial construction permitted since 2011, from heliports to high-rise buildings, and for single-family houses since 2007. These rules now apply to all construction statewide in Washington. With hundreds of buildings now tested, we interviewed several envelope consultants and contractors to elicit advice for their colleagues in other jurisdictions that initiate mandatory air barrier testing. Seattle’s specific code requirements are explained, including the “testing mandatory, passing optional” provision, and stakeholder suggestions for clarifying some code provisions are discussed. In addition, the impacts of air barrier testing on construction scheduling and costs are discussed, and the degree of leakage reduction achieved is estimated. The paper includes Seattle’s “Top 10 Lessons Learned” (what we wish we’d known before we started) and the “Top 10 Air Leaks” commonly discovered during testing. Additional significant issues discussed include the influence on test results of the quality of the air barrier itself versus the quality of test preparation, and the issues that were experienced by the various stakeholders as they adapted to the new code requirements. The paper concludes with consideration of potentials for strengthening of air barrier testing provisions in future code cycles.