Although health literacy is commonly defined as an individual trait, it does not depend on the skills of individuals alone. Health literacy is the product of the interaction between individuals' capacities and the health literacy-related demands and complexities of the health care system. Specifically, the ability to understand, evaluate, and use numbers is important to making informed health care choices. Health Literacy and Numeracy is the summary of a workshop convened by The Institute of Medicine Roundtable on Health Literacy in July 2013 to discuss topics related to numeracy, including the effects of ill health on cognitive capacity, issues with communication of health information to the public, and communicating numeric information for decision making. This report includes a paper commissioned by the Roundtable, "Numeracy and the Affordable Care Act: Opportunities and Challenges," that discusses research findings about people's numeracy skill levels; the kinds of numeracy skills that are needed to select a health plan, choose treatments, and understand medication instructions; and how providers should communicate with those with low numeracy skills. The paper was featured in the workshop and served as the basis of discussion.