This chapter focuses on the Advocacy Coalition Framework (ACF), a highly influential approach to public policy that has been adopted in the United States and many other developed countries. The ACF first emerged in the 1990s, seeking to produce a general theory of policy-making based on the idea that people engage in politics to translate their beliefs, rather than their simple material interests, into action. In addition to representing an approach to the study of contemporary public policy, the ACF offers a set of ideas about how scientific enquiry should be conducted. The chapter examines how the ACF’s proponents, especially Paul Sabatier, have influenced the discussion of public policy as a discipline and a branch of science. After providing a summary of the ACF and its ideas, the discussion turns to its expansion and revision and how it has shaped thinking and research on public policy.