In 2008 Thaler and Sunstein coined the concept ‘nudge’ in their book carrying the same name. Since then the concept of nudge as well as the derivate concept ‘nudging’ have been main drivers in the emergence of Behavioural Public Policy. From the outset, however, confusion has reigned as to what exactly is to be understood by these two concepts and their role within Behavioural Public Policy with direct implications for its evaluation as an acceptable policy approach. This chapter provides an introduction to the theoretical foundations of behavioural public policy, a revised definition of nudge that cleans up conceptual mess and locates nudging amongst three strings of behavioural public policy: push, clear and nudge. Finally, ‘nudging’ is defined as the systematic and evidence-based development and implementation of nudges in creating behaviour change and some concerns about nudging in public policy are addressed.