In an effort to make urban transportation more sustainable, policy makers and social marketers have turned increased attention to bicycling. The authors apply behavioral reasoning theory to investigate consumers' mental processing of bicycle commuting to better understand why such an active mode of transportation is underutilized in many urban areas. Using a large-scale sample of commuters in Dublin, Ireland, the study offers insights into the psychological antecedents of bicycling adoption. The structural equation modeling results confirm the main premise of behavioral reasoning theory, in that reasons serve as important linkages between people's values and their attitudes and behavior. In particular, the findings suggest that commuters have reasons not to engage in bicycling (e.g., inconvenience, perceived clanger), which account for most of the variance in bicycle-commuting behavior. The findings challenge interventions that emphasize the role of information campaigns and one-time infrastructural changes in encouraging voluntary change in travel behavior. Instead, the findings suggest that transportation planners and social marketers would be better off addressing consumers' reasons for and against bicycling through community-based initiatives.