Abstract
This article describes the importance of the role of mathematical models in the study of travel behavior over the past 30 years. Early ideas about models have become standard tools in today's research. The author suggests there is still much work to be done to enhance, extend, and integrate developments of the past 30 years into development of new models. Five areas for new research are briefly outlined, including developing an integrated system of models to build on new knowledge about human decision-making, development patterns and transportation system operations.