In this paper we put forward a theory of large systems change (LSC), where large systems are defined as having breadth (i.e. engaging large numbers of people, institutions, and geographies) and depth (i.e. changing the complex relationships among elements of power and structural relationships simultaneously). We focus primarily on transformational LSC, recognising that such systems are complex adaptive systems in which change is continuous and emergent, but directions can be supported. A typology of change actions with two core dimensionsconfrontation and collaboration on the horizontal axis and generative and ungenerative change on the verticalsuggests that change strategies can be classified into four broad archetypes: forcing change, supporting change, paternalistic change, or co-creating change. LSC theory development focuses on three core questions: what is the foundation of LSC concepts and methods, what needs to change, and how does LSC occur? We conclude by reviewing how papers in the Special Issue fit into these questions.