Attached garages are a staple of modern convenience. They allow access to and from the living space without exposure to the elements, and they keep vehicles and other contents warmer in cold weather than their detached counterparts. As such they are a sought after feature in both the real estate and new construction markets.For all their conveniences, attached garages can pose a threat to a home’s indoor air quality. Carbon monoxide from internal combustion engines is poisonous at moderate concentrations, and effects from chronic exposure to volatile organic compounds from chemicals such as pesticides, paints, and other frequently garaged items are likely detrimental. These contaminants and their byproducts can migrate across garage-house interfaces through bypasses in the structure, or via ductwork and HVAC equipment present in garages.This paper presents results from an ASHRAE-sponsored project on the migration of garage contaminants into the home in five houses in central Illinois with a variety of attached garage configurations. Three of the houses had HVAC equipment present in the garage or in an adjacent connected space, and two had living space directly over the garage. Three were measured over the heating season, and two were measured over the cooling season, with one of those receiving supplemental baseline testing during the preceding heating season.Carbon monoxide and carbon dioxide from vehicle exhaust and intentionally introduced sulfur hexafluoride tracer gas as a surrogate for general garage air were measured at approximately 12 minute intervals from two locations within the garage, and from a variety of locations within the living space and garage-adjacent attic and foundation spaces.Over the course of each multi-week field investigation, progressive interventions were completed including implementation of temporary passive ventilation, air sealing at the house-garage interface, air sealing of ductwork in the garage (when applicable), and mechanical ventilation operating at multiple speeds and under various control strategies. The magnitude of contaminant transport, and the impacts of these interventions are analyzed and discussed.