In the short, turbulent history of AIDS research and treatment, the boundaries between scientist insiders and lay outsiders have been crisscrossed to a degree never before seen in medical history. Steven Epstein's astute and readable investigation focuses on the critical question of "how certainty is constructed or deconstructed," leading us through the views of medical researchers, activists, policy makers, and others to discover how knowledge about AIDS emerges out of what he calls "credibility struggles." Epstein shows the extent to which AIDS research has been a social and political phenomenon and how the AIDS movement has transformed biomedical research practices through its capacity to garner credibility by novel strategies. Epstein finds that nonscientist AIDS activists have gained enough of a voice in the scientific world to shape NIH–sponsored research to a remarkable extent. Because of the blurring of roles and responsibilities, the production of biomedical knowledge about AIDS does not, he says, follow the pathways common to science; indeed, AIDS research can only be understood as a field that is unusually broad, public, and contested. He concludes by analyzing recent moves to democratize biomedicine, arguing that although AIDS activists have set the stage for new challenges to scientific authority, all social movements that seek to democratize expertise face unusual difficulties. Avoiding polemics and accusations, Epstein provides a benchmark account of the AIDS epidemic to date, one that will be as useful to activists, policy makers, and general readers as to sociologists, physicians, and scientists.