Transportation planners are increasingly adopting policies aimed at changing travel choices made by citizens. Rather than trying to solve transportation problems by building highways and transit routes, "transportation demand management" relies on incentives and disincentives to promote carpooling, vanpooling, transit use and changed work hours. These approaches attempt to accommodate travel demand by more efficiently utilizing existing facilities. While many argue that transportation behavior cannot be changed, this review demonstrates that many years of behavioral science research on travel show otherwise. Commuters respond to differences in travel time and travel cost, to changes in work hours and other attributes of travel in systematic ways which are quite predictable. As a consequence, travel demand management is a promising approach to regional transportation planning.