Critical realism is a philosophy of science that is increasingly occupying the center of discussion in the theory of International Relations. The most important aspect of critical realism is that it shifts the focus of controversy in international relations from epistemology to ontology. According to the materialist ontology of critical realism there exists a reality independent of our observations and experiences. This is an alternative to the dominant positivist as well as post-positivist conceptions of science which associate reality either with what can be observed or with what can be said and thought in discourse. Critical realism provides an understanding of science that overcomes the difficulties of both and explains international relations as part of a totality of social relations with varying ontological depths. By defining structures in terms of social relations, critical realism presents a structural analysis of international relations different from the structuralism of neorealism and develops a transformational model of social activity which tries to avoid both the voluntarism of individualist/unit based analyses and the determinism of structuralist analyses.