Available evidence establishes correlations between the built environment and walking, but not a causal relationship, leading researchers to debate whether "self-selection" explains the observed correlations: do residents who prefer to walk choose to live in more walkable neighborhoods? Using data from a survey of residents of eight neighborhoods in Northern California, this article presents new evidence on the possibility of a causal relationship between the built environment and walking behavior. This work improves on most previous studies by incorporating travel attitudes and neighborhood preferences into the analysis of walking behavior, and by using a quasi-longitudinal design to test the relationship between changes in the built environment and changes in walking. Both analyses show that the built environment has an impact on walking behavior even after accounting for attitudes and preferences.