The major goal of travel behavior research is to understand and model the processes by which people make decisions regarding activities and travel. These decisions, including whether, when, where, and with whom to participate in particular activities, and the choice of mode and route, are collectively known as activity scheduling decisions. Traditionally the research focus has been on how these decisions are made, how they relate to demographic and environmental factors, and, to a lesser extent, how they interact with each other. A rich body of knowledge, increasingly useful for forecasting behavioral responses to a wide range of transportation and other policies, is already part of this tradition.