Numeracy--that is, how facile people are with mathematical concepts and their applications--is gaining importance in medical decision making and risk communication. This article proposes six critical functions of health numeracy. These functions are integrated into a theoretical framework on health numeracy that has implications for risk communication and medical decision-making processes. The authors examine practical underpinnings for targeted interventions aimed at improving such processes as a function of health numeracy. They hope that the proposed functions and theoretical framework will spur more research to determine how an understanding of health numeracy can lead to more effective communication and decision outcomes. (Contains 1 figure.)