This chapter provides the conceptual foundation for the book by clarifying what is meant by public policy, evidence, EBPM. It first introduces the idea of public policy and the policymaking process, highlighting how governments pursue societal goals through a series of stages. It then defines EBPM and reviews how it has been promoted and practiced in Japan and in other countries. Building on this, the chapter synthesizes key themes in the EBPM literature, including what counts as evidence, how evidence is produced and used at different stages of the policy process, and how political and institutional factors shape the implementation of EBPM in practice. Together, these discussions establish the analytical lens through which the subsequent chapters examine EBPM in Japan and the UK and explore the potential of Big Data to support policymaking.