The theory-theory account of conceptual development posits that children's concepts are integrated into theories. Concept-learning studies have documented the central role that theories play in children's learning of experimenter-defined categories but have yet to extensively examine complex, real-world concepts, such as health. The present study examined whether providing young children with coherent and causally related information in a theory-based lesson would facilitate their learning about the concept of health. This study used a pretest/lesson/posttest design, plus a 5-month follow-up. Children were randomly assigned to 1 of 3 conditions: theory (i.e., 20 children received a theory-based lesson), nontheory (i.e., 20 children received a nontheory-based lesson), and control (i.e., 20 children received no lesson). Overall, the results showed that children in the theory condition had a more accurate conception of health than children in the nontheory and control conditions, suggesting the importance of theories in children's learning of complex, real-world concepts. (Contains 4 tables.)