A decade-long study of young adulthood in Malawi that demonstrates the impact of widespread HIV status uncertainty, laying bare the sociological implications of what is not known. An Epidemic of Uncertainty advances a new framework for studying social life by emphasizing something social scientists routinely omit from their theories, models, and measures–what people know they don’t know. Taking Malawi’s ongoing AIDS epidemic as an entry point, Jenny Trinitapoli shows that despite admirable declines in new HIV infections and AIDS-related mortality, an epidemic of uncertainty persists; at any given point in time, fully half of Malawian young adults don’t know their HIV status. Reckoning with the impact of this uncertainty within the bustling trading town of Balaka, Trinitapoli argues that HIV-related uncertainty is measurable, pervasive, and impervious to biomedical solutions, with consequences that expand into multiple domains of life, including relationship stability, fertility, and health. Over the duration of a groundbreaking decade-long longitudinal study, rich survey data and poignant ethnographic vignettes vividly depict how individual lives and population patterns unfold against the backdrop of an ever-evolving epidemic. Even as HIV is transformed from a progressive, fatal disease to a chronic and manageable condition, the accompanying epidemic of uncertainty remains fundamental to understanding social life in this part of the world. Insisting that known unknowns can and should be integrated into social-scientific models of human behavior, An Epidemic of Uncertainty treats uncertainty as an enduring aspect, a central feature, and a powerful force in everyday life.