In recent years, the science of behavioural economics is becoming mainstream; policymakers increasingly embrace behaviourial insights in order to improve their policies (e.g. Lunn, 2012; Van Bavel et. al., 2013; World Bank, 2015). They use various behavioural theories, specific tools (e.g. nudges), and create new tasks, functions and organizations (e.g. Behavioural Insight Teams). As a consequence the somewhat new and fuzzy job practice of behavioural administration is emerging, promising innovative, cheaper, and more effective policy interventions (e.g. Thaler and Sunstein, 2008). Against this background, this study analyzes how behavioural administrators seek to advance and institutionalize their new practice inside governments. We interpret this process as an attempt at professionalization (Freidson 2001; Noordegraaf et. al., 2014), in which behavioural administrators are carving out the internal practices of behavioural policymaking as well as the external linkage of these practices to established participants and procedures of policymaking. The paper is based on ethnographic research with interviews with and observations of behavioural administrators at Dutch central government bodies. Our analytical focus lies on both the internal professionalization logic, identifying how behavioural administrators construct exclusive identities, knowledge and standards, as well as the external logic, describing how they organize their position in the organization while managing relations with colleagues, bosses and external peers in order to obtain institutional control (cf. Noordegraaf, 2007; Freidson, 2001). Our analysis demonstrates how the advancement of behavioural administration is fragmented, with respondents using differing behavioural theories and (sub)disciplines, (methodological) standards and organizational embeddings. At the same time, they have a shared identity as newcomers, struggling to get their ‘behavioural message’ across and attain organizational legitimacy and effectiveness. That struggle is nonetheless handled and familiarized by framing it as a matter of behavior change: an opportunity for behavioural administrators to practice what they preach, gain occupational control, and bring ‘Nudge’ forward.