Chapter 2 gives an overview of the philosophical foundations of the conceptual development of complex adaptive systems (CAS). This provides some grounding for understanding the nature of intertwined social-ecological systems (SES). The chapter offers a brief discussion on the history of ideas and how the discovery of the features and behaviour of CAS introduced a shift in worldview for modern science. We then elaborate on why the study of SES as CAS calls for new methods and more integrated and collaborative research approaches. Finally, the implications for action in responding to the current grand challenges of securing more sustainable SES futures are discussed.