Nudging has been used to make public policies widely, in various fields such as personal finance, health, education, environment/climate, privacy, law, and human well-being. Nonetheless, with an increase in the applications of nudging, the toolkit of nudges also expanded massively, which ultimately led to multiple different conceptualisations and definitions of the nudge. In this entry, we review developments to nudge and nudging in public policy. First, we briefly discuss the political philosophy and psychological paradigm behind the conventional nudge, and examples of economically modelling nudge applications. Then, we highlight the role of nudges in behavioural public policy, an emerging subdiscipline of public policy which uses insights from behavioural sciences to develop new policies. We review the many definitions of nudge and introduce alternative toolkits of behaviours change, such as thinks, boosts, nudge+. We conclude with a discussion on the limitations of nudging in public policy and future research in behavioural public policy.