Sociologists, philosophers, and historians of science tend to focus their attention on the production of knowledge. More recently, scholars have begun to investigate more fully the structures and processes that impede the production of knowledge. This article draws on interviews conducted with 41 academic researchers to present a phenomenological examination of "forbidden knowledge"—a phrase that refers to knowledge considered too sensitive, dangerous, or taboo to produce. Forbidden knowledge has traditionally been understood as a set of formal controls on what ought not be known. We argue that the social processes that create forbidden knowledge are embedded in the everyday practices of working scientists. The narrative legacies of past controversies in science are of particular importance, as they serve as a tool that working scientists use to justify, construct, and hide their acceptance of forbidden knowledge. As a result, the precise contents of forbidden knowledge are fluid, fuzzy, essentially contested, specialty specific, locally created, and enforced.