In brief: The author reviews the key research on the physiological determinants of distance running performance and presents a simple model that he thinks should serve as a basis for hypothesis testing in the future. The model has three primary determinants: aerobic capacity relative to body weight, running economy, and the percentage of aerobic capacity used during a race. The author says that the model applies equally to elite runners and recreational joggers and suggests that systematic efforts are needed to translate research findings to a form that is meaningful to clinicians, coaches, and athletes.