Background: Walking and cycling for transport, or 'active travel,' has the potential to contribute to overall physical activity levels. However, a wide range of factors are hypothesized to be associated with adult's active travel behavior. This paper describes current knowledge of the psychological and environmental determinants of active travel in adults, and considers ways in which the 2 domains can be better integrated. Methods: Quantitative studies were reviewed which examined psychological and environmental infuences on active travel in an adult population. Studies were classifed according to whether they examined psychological, environmental or both types of factor. Results: Fourteen studies were identifed which examined psychological correlates of active travel behavior in adults, and 36 which examined environmental correlates. Seven studies were identifed which considered both domains, of which only 2 of explored the interactions between personal, social and environmental factors. The majority of the evidence is helpful in identifying correlates rather than determinants of active travel behavior. Conclusions: To further our understanding of the infuences of active travel, there is a need for more research which integrates both individual and environmental domains and examines how they interact.