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. This investigation focuses on the critical question of "how certainty is constructed or deconstructed", leading readers through the views of medical researchers, activists, policy makers and others to discover how knowledge about AIDS emerges of what the author calls "credibility struggles". The author 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 non-scientist 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 democractize medicine, 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.