Contemporary policy work is deeply informed by the circulation of policy initiatives and models from other jurisdictions. Sometimes close and at other times distant, the influence of various 'elsewheres' (Allen and Cochrane, 2007) has become a routine feature of the policy process. Researchers from a range of academic disciplines have matched the increased traffic of policy knowledge with a growing body of knowledge that seeks to document and understand it. While political scientists have the longest history of engagement with travelling policy, anthropologists, geographers, sociologists, urban planners and other social scientists have joined the fray in recent years, creating a lively multidisciplinary research effort (Benson and Jordan, 2011; McCann and Ward, 2012). Yet, for the most part, this research effort has been disjointed. Despite having a common interest at their core (the movement of policy), there are largely separate conversations occurring, structured around concepts such as policy diffusion, policy learning, policy transfer and policy mobility. These conversations have 'lived together apart' for some time now, but appear to be converging toward a focus on diverse arenas, agents and actions implicated in the circulation of policy. We have also seen shifts from structure-agency binaries to notions of contextually-embedded agency, from neat, spatially and temporally delimited processes to messy, ongoing processes, and from an exclusive interest in the 'why' of travelling policy to a broader set of research questions regarding the 'how'. In this book we use 'policy circulation' as an expedient umbrella term that signifies this emergent zone of common ground. We intend the notion of policy circulation to be largely agnostic (in ontological, epistemological and methodological terms), but it is inescapably oriented toward the work involved in moving policy and the ongoing nature of such efforts. It provides a loose conceptual and empirical space in which Policy Diffusion Policy diffusion emerged as a distinct area of inquiry in the 1960s, put to the task of studying patterns of intra-national policy adoption in the context of United States federalism. Policy diffusion studies seek to map and explain sequential patterning related to the uptake of 'policy innovations'-defined by Walker (1969, p. 881) as 'a program or policy which is new to the state adopting it'-to understand when and why certain jurisdictions adopt policies from other jurisdictions (see Simmons and Elkins, 2004; Levi-Faur, 2005; Simmons et al., 2006). Although the term 'policy diffusion' is often invoked to describe 'a process through which policy choices in one country affect those made in a second country', Marsh and Sharman (2009, pp. 270-71) argue that those researching policy diffusion more often have in mind 'a process of interdependent policy convergence'. Here, the choices of governments are not just interdependent, whereby 'the choice of a government influences the choices made by others and, conversely, the choice of a governmen...