This paper argues that the much discussed issue between "scientific realism" and "instrumentalism" has not been clearly drawn. Particular attention is paid to the claim that only realism can "explain" the success of scientific theories and---more especially---the progressively increasing success of such theories in a coherent line of inquiry. This claim is used to attempt to reach a clearer conception of the content of the realist thesis that underlies it; but, it is here contended, that attempt fails, and the claim (...) itself hangs in the air. A series of increasingly sophisticated versions of the "instrumentalist" thesis is considered, and both these and the contentions of realism are placed in relation both to particular examples of scientific development and positions historically maintained by philosophers and by scientists. The author’s conclusion is that, when the positions are assessed against the background of the actual history of science, each of the contrary doctrines, interpreted with excessive simplicity, is inadequate as a theory of the dialectic of scientific development; each, so interpreted, has contributed in important instances to actual damage to investigations by great scientists ; whereas in both the theoretical statements and the actual practice of the most sophisticated philosophers/scientists, important aspects of realism and instrumentalism are present together in such a way that the alleged contradiction between them vanishes. (shrink)