A `Behavioural Insights’ movement has entered the global policy scene. Drawing from the behavioural sciences, this movement has been pushing novel forms of policy analysis and design, particularly promoting the use of `nudges’ and field experiments. Despite Behavioural Insights’ apparent popularity, its contribution to state and society has been appraised divergently. Some see a promise of radical evidence-based government, others a looming manipulative technocracy, and again others a trivial fad. In light of the puzzle posed by these clashing appraisals, this study has explored Behavioural Insights from up close. Based on ethnographic fieldwork in Dutch government, it asks what Behavioural Insights experts actually do, how they are professionalizing and how they `see’. The result is a rich account of the emerging `behavioural state’, with a sharper understanding of its varieties, tensions and ambiguities.