This chapter offers critical realism as a philosophical and general theoretical approach, which provides a coherent and convincing underpinning for interdisciplinary thinking and research. The approach offered in this book to underpin interdisciplinary thinking and research is provided by critical realism. Critical realism is argued to be uniquely placed to provide a coherent basis and tools for justifying and practising integrative interdisciplinary research through embracing ontological issues. Critical realists emphasize cause and the causal powers and liabilities of objects in the natural and social world. Emergence is a key concept for critical realism. It provides the basis for causal efficacy at high levels of order. The final feature of critical realism considered in this chapter concerns the relationship between knowledge and social change. Critical realism provides an approach to overcome these problems by foregrounding ontology - providing a view of natural and social objects and structures, their causal relations and emergence.