Changes in the built environment and travel patterns in the Unites States and other countries over the last several decades may in part be responsible for declines in physical activity among adults and children. There is increasing interest in understanding the determinants of transport-related physical activity, or active travel, as a way to reverse this trend. In particular, active travel to school and to public transit have been identified as modifiable behaviors that vary as a function of environmental and individual-level factors. To advance the scientific understanding of active travel, there will need to be a focus on interdisciplinary research teams, more consideration of measurement issues, and an emphasis on acquiring longitudinal data.