Publisher Summary This chapter discusses the demarcation of science from non-science. If a discipline were unable to offer a characterization of its subject matter, it is suspected that its representatives do not really know what they are talking about. Evidently, what holds for all ﬁelds of inquiry also holds for a particular discipline such as the philosophy of science. Therefore, it belongs to the job description, so to speak, of the philosopher of science to tell what the “thing” called “science” is. Whereas everyone seems to know intuitively which ﬁelds of knowledge are scientiﬁc (such as physics and biology) and which are not (such as astrology and palmistry), it has proved difficult to come up with a satisfactory demarcation criterion. Indeed, many of the demarcation criteria proposed by philosophers of science have proved to be unsatisfactory, for being either too narrow or too wide.