Chapter 1 introduces the concept of social-ecological systems (SES) as intertwined systems of people and nature, and discusses the key features of SES as complex adaptive systems. The chapter then provides an overview of SES research, including a brief history of the origins of the field and how it relates to other similar fields. The key conceptual frameworks used in SES research, and some of the most commonly used approaches, are introduced. The chapter concludes with a discussion of the links between research and action in SES research and how these are interwoven in many SES research contexts, particularly those aimed at supporting and facilitating systemic change towards more sustainable futures.