Why is science so credible? Usual answers centre on scientists' objective methods or their powerful instruments. This text argues that a better explanation for the cultural authority of science lies downstream, when scientific claims leave laboratories and enter courtrooms, boardrooms, and living rooms. On such occasions, we use maps to decide who to believe - cultural maps demarcating science from pseudoscience, ideology, faith, or nonsense. Thomas F. Gieryn looks at episodes of boundary-work: Was phrenology good science? How about cold fusion? Is social science really scientific? Is organic farming? After centuries of disputes like these, Gieryn finds no stable criteria that absolutely distinguish science from non-science. Science remains a pliable cultural space, flexibly reshaped to claim credibility for some beliefs while denying it to others. In an epilogue, Gieryn finds this same controversy at the heart of the raging science wars.