This chapter introduces the concept of policy accumulation and describes the measurement approach we employ in order to investigate the phenomenon. Our conception crucially departs from dominant accounts of policy change, which typically analyse policy change from a perspective focusing on individual policies. In contrast, our concept of policy accumulation captures the aggregate consequences of these individual policy developments over time. This change in perspective constitutes one of the central innovations suggested in this book. The purpose of this chapter is thus threefold. First, we will specify in more detail the conceptual challenges that are associated with an assessment of policy accumulation. In so doing, we build on existing aggregate concepts in the public policy literature. Second, we introduce our concept that builds on the analysis of policy targets and policy instruments as central elements and universally applicable units of policy accumulation. In a third step, we provide more information on the concrete operationalization of policy accumulation, including the specification of indicators as well as aspects measurement and data collection.