Although health and illness concepts are thought to be important mediators of health and illness behaviors, little work has been done examining health concepts, and no research has examined the two conceptual structures simultaneously. This study examined concepts of health and illness in 218 adolescents between the ages of 11 and 18 years. Results were consistent with the hypothesis that concepts of health and illness are not opposite ends of a single health dimension, but reflect different and overlapping constructs. The degree of overlap varied as a function of age, with health concepts showing less emphasis on "the absence of illness" with greater maturity. The results suggest that models of health behavior that focus on illness avoidance neglect many components of health salient to lay persons.