The study of cases is a long established element of social researchers’ methods. This chapter discusses the significance of complexity theories for case study, their implications in terms of methodology, methods, and analysis, and the affordances and limitations flowing from adopting a complexity perspective. Different perspectives on using complexity concepts are considered and the kind of insights they offer. In this chapter, we adopt an understanding of complexity in social systems that sees large scale, global patterns as emerging from interactions of individuals and groups at different levels. As a consequence, this understanding recognizes the researcher as a participant in complex, systemic processes rather someone who stands outside and observes whole systems. I discuss cases and case study in the light of key complexity concepts. There is a focus on the kind of insights and knowledge available through case study, definitions of cases and their boundaries in light of the open nature of complex systems, and methods that lend themselves to gaining insights into patterns that persist and change over time. In doing so, I illuminate the dynamic, shifting relationships underlying those patterns. I provide examples of studies that apply complexity thinking to cases at different scales, from individual schools to transnational comparisons of school systems. I conclude this chapter by discussing the insights and affordances of the complexity perspective including that of revealing complex processes and interactions that lead to emergent outcomes, and the challenge to the limitations of knowledge of a complexity mindset.