This chapter provides the theoretical framework for the book. It starts with definitions of the key terms ‘international development’ and ‘unintended effects.’ It then argues for a complexity thinking framework when dealing with unintended effects, as opposed to the linear thinking that most existing development programming is based on. It explains the key concepts of complexity thinking: feedback loops, non-linearities, interconnectivities, alternative impact pathways, and adaptive agents. It further illustrates the mechanisms of the complexity framework through a case study of the unintended effects of due diligence legislation on conflict minerals as part of the Dodd-Frank Act of 2010. The chapter concludes by explaining the persistence of unintended effects with the three limits to policy learning, which may prevent organizations from effectively dealing with unintended effects: ideological, institutional, and technical limits.