"Numeracy," so-called on analogy with literacy, is essential for making health and other social judgments in everyday life [Reyna, V. F., & Brainerd, C. J. (in press). The importance of mathematics in health and human judgment: Numeracy, risk communication, and medical decision making. "Learning and Individual Differences."]. Recent research on numeracy in health decision making has shown that many adults fail to solve simple ratio and decimal problems, concepts that are prerequisites for understanding health-relevant risk communications. In addition, adults exhibit a ratio bias, in which higher frequencies bias probability judgments, and denominator neglect, described by Reyna and Brainerd (e.g., [Reyna, V. F. (1991). Class inclusion, the conjunction fallacy, and other cognitive illusions. "Developmental Review, 11", 317-336.; Reyna, V. F., & Brainerd, C. J. (1994). The origins of probability judgment: A review of data and theories. In G. Wright & P. Ayton (Eds.), "Subjective probability." (pp. 239-272). New York: Wiley.]) and independently by Epstein (e.g., [Epstein, S. (1994). Integration of the cognitive and psychodynamic unconscious. "American Psychologist, 49", 709-724.]). Along with research in education and cognitive development, this work demonstrates that adults have difficulty with a broad range of ratio concepts, including fractions, proportions, risks and probabilities. The psychological mechanisms underlying this difficulty are characterized using dual-processes approaches such as fuzzy-trace theory, simple and effective interventions are described that eliminate common problem-solving errors, and implications for the effective use of numerical information in risk communication are discussed.