Over the past decade behavioural public policy has spread inter- and transnationally. Despite an ever-growing amount of literature, however, there are only few systematic studies on the historical roots and developments of the behavioural change agenda, the varieties of behavioural public policy, the causes for its inter- and transnational spread as well as its normative implications and political consequences. The handbook contributes to addressing this gap by mapping the terrain and bringing together scholarship from a wide variety of conceptual and analytic perspectives. This introductory section gives an overview on the main topics, the individual chapters and on future research perspectives. We suggest tracing the multiple ways behavioural public policy is constituted, translated and legitimized across policy areas and jurisdictions. This, in turn, is a precondition for understanding its role in the transformation of public policy.