A few years ago, Bas van Fraassen reminded philosophers of science that there are two central questions that a theory of explanation ought to answer. First, what is a (good) explanation—when has something been explained satisfactorily? Second, why do we value explanations? (van Fraassen, 1977, 1980, ch. 5). For a long time, discussions of explanation concentrated on technical problems connected with the first of these questions, and the second was by and large ignored. But, in fact, I think it is the second question which raises the more fundamental and interesting philosophical issues. I shall offer reasons for thinking that the answer to the first question requires acceptance of the sort of fullblown notion of causation that only a scientific realist can love, and that the answer to the second question requires a realist construal of scientific theories and scientific methodology. My argument will be mainly negative, surveying the problems facing some major alternative accounts of explanation. A full elaboration of the realist perspective will have to await the completion of work in progress.