Dissertation Summary: Active transportation modes (walking, bicycling, and transit) have the potential to help mitigate environmental and health concerns ranging from growing greenhouse gas emissions to increasing rates of physical inactivity and obesity. Hundreds of studies over the last three decades suggest that built environment characteristics such as land use mix, street connectivity, and access to pedestrian, transit and bicycle infrastructure are associated with rates of active travel. The proposed research attempts to move beyond questions of whether built environment characteristics influence travel mode decisions to instead explore how the built environment affects these choices. Using Ajzen's theory of planned behavior, I will model the direct and indirect effects of the built environment on the proximate causes of behavior: attitudes, perceived behavioral control, social norms, and behavioral intention. Data in the model will come from a two-wave panel survey of recent movers in Portland, Oregon. New movers are an important, yet often overlooked, population in travel behavior research because they provide an opportunity to observe behavior adoption in new contexts and because the roughly one-in-ten Americans who move each year are more likely to consider changes to their daily routines, making them an ideal target population for efforts to shift travel behavior.