Problem, research strategy, and findings: Planners need a clear understanding of what influences walking and bicycling behavior to develop effective strategies to increase use of those modes. Transportation practitioners have largely focused on infrastructure and the built environment, although researchers have found that attitudes are also very important. The theory of planned behavior (TPB) suggests that behavior such as active transportation results from a mixture of personal attitudes toward these modes, subjective norms, and a person's perceived behavioral control, giving us a way to conceptualize psychological factors that influence travel behavior. Using data from a random phone survey of three neighborhoods in Portland (OR), we test whether TPB explains the possible causal relationships among the built environment, socio-demographics, and active transportation. We find that both the built environment and demographics influence cycling and walking, although indirectly, by influencing attitudes and perceived behavioral control. Moreover, it is important to look at bicycle-specific infrastructure separately from other environmental characteristics. For example, relatively flat neighborhoods with well-connected, low-traffic streets and multiple destinations were associated with more frequent bicycling, but striped bike lanes were not. Takeaway for practice: Practitioners cannot rely solely on changing the environment to increase bicycling. Programs such as public events and individualized marketing that influence attitudes may be necessary to reinforce positive environmental features. This is particularly true for women and older adults. Moreover, adding bike lanes to an otherwise poor bicycling environment may not increase bicycling in any significant way.