As both energy modelers and whole building air infiltration testers, we often are asked how to account for known air infiltration in an energy model. The answer is not simple or easy, and from the number of questions we have received over the years, infiltration in energy models is not well understood by either energy modelers or those evaluating model results. The impact of infiltration assumptions is so variable that many energy models for code compliance or energy use certification prescribe which infiltration values to use to simplify the evaluation of model results. Sure, a window replacement company will guarantee you a winter fuel savings after installing their new windows in your home. They probably do not lose much of their profit if they miss the target for your first winter and have to make a partial rebate to you, or they still would not be in business. It is a different proposition when a commercial building client wants reliable construction cost and energy-savings data to make a long-term economic decision for a project. The model assumptions and limitations need to be well understood by all parties to avoid contentious relations at project completion. This paper reviews the way infiltration is accounted for in various energy models and explains how actual test data or target air tightness can be accounted for in those energy models. Using our investigation experience, we explain how mechanical system operations and operable windows can affect infiltration and energy use predictions. Finally, we compare infiltration simulation strategies in energy models to demonstrate the typical variability in results. We include some project energy models calibrated to actual measured infiltration and building energy performance to further illustrate potential modeling impacts.