Despite a wide body of literature on the importance of program theory and the need to tackle complexity to improve international development programming, the use of program theory to underpin interventions aimed at facilitating change in complex systems remains a challenge for many program practitioners. The actor-based change framework offers a pragmatic approach to address these challenges, integrating concepts and frameworks drawn from complexity science and behavioral change literature to develop robust program theory for complex interventions. This article presents the conceptual framework for the approach and describes how it has been applied in practice on an evaluation of a security and justice program in Nepal. It concludes with a discussion of the strengths and weaknesses of the approach in practice and how it can be applied more widely to improve program theory for interventions in complex systems.