Governments are increasingly embracing behavioural science to improve their policies, reflected in the rise of the so-called ‘Behavioural Insights’ movement. Based on half-open interviews with these upcoming Behavioural Insights experts in Dutch government, this chapter describes how they are developing their occupation, as a possible attempt to professionalize. The key issue for us is whether behaviour experts are uniform or fragmented. Our findings reveal that they are both. On the one hand, behaviour experts are unified in that they all apply behavioural science to policy-making, and to some extent rely on similar theories, methods, tools, and knowledge platforms. However, beyond these basic uniformities, behaviour experts are widely fragmented. As they are still at an explorative phase of development, possess high degrees of autonomy, and draw from a broad body of knowledge, we expect behaviour experts to remain a fragmented group in the near future.