This chapter is divided in seven sections, based on the chapters contained in this handbook. Section 1 presents the policy design framework based on four dimensions –causation, evaluation, instrumentation and intervention– with their corresponding activities and causal mechanisms. In section 2 we delve into the theories of policy design around three common threads: the politics of policy design, policy instrumentation and social constructivism. In section 3 we deal with the main problems of causation, namely the multiple causes of policy problems and bounded rationality in decision-making. In section 4 we proceed with the problems raised by evaluation, framing and values, considering the importance of ideas and cognitive frames, on one hand, and policy change through re-design, on the other hand. In section 5 we discuss the core aspects of instrumentation and coordination related to governance, institutional frames and coordination mechanisms. In section 6 we present a series of innovations in contemporary models of intervention, such as design thinking and policy labs, or deliberation and co-design. The concluding section draws on the future of policy design regarding complexity, expertise and collaboration in the four-fold model of our framework.