Individuals working within the health sector widely embrace the idea of using evidence to achieve their goals of improving individual and population health. Yet while these actors embrace an ideal form of rational-instrumental evidence use under the banner of ‘evidence based policymaking’, they often struggle to understand when, why, or how evidence is used in policy processes. This chapter sets out the conceptual framework employed in this volume to study the use of evidence within policymaking from a public policy perspective. It explores the importance of both political contestation and institutional context to understand when and how evidence will be used within policy processes. The chapter then outlines the structure of this book and the focus of subsequent chapters, highlighting how each of these talks to these themes.