Rationality, Relativism and Incommensurability is a collection of essays on a number of interconnected topics in the philosophy of science. The essays focus on problems arising from the introduction of an 'historical' or 'post-empiricist' perspective on the philosophy of science. These problems include the issues of the semantic incommensurability of scientific theories, the rationality of choice between theories, the variation of methodological criteria, and the recent turn to naturalised approaches to the justification of methodological criteria. The underlying aim of the book is to show how the phenomenon of extensive conceptual and methodological variation in science need not give rise to a thorough-going epistemic or conceptual relativism.