The understanding of science by members of the public has been of increasing concern to social scientists. This article argues that such understanding, or the ostensible lack of it, is structured by discourses that address science both as an abstract entity or principle and as an activity directed at specific phenomena or problems. Drawing upon a wide range of interviews about various sources of ionizing radiation, it is suggested that understanding is tied to questions of social identity that encompass relations (...) of differentiation from and identification with science and the institutions in which it is embedded. (shrink)