The Dutch Central Planning Bureau (CPB) is the most prominent scientific advisory body of the Dutch government on economic issues. By providing a macroeconomic framework the CPB plays an important role in the preparation and coordination of social and economic policy. With the figures and forecasts it offers to national media and politicians, the CPB is also an influential actor in public debates. In both instances, the expertise that the bureau offers is strongly based on the macroeconomic models it employs. By discussing two periods in the history of the CPB, this article will investigate how the CPB gained this important role in Dutch politics and what the relation is between its modelling practices and economic expertise. The first period deals with the CPB’s initial attempts of gaining a foothold in policymaking circles from 1945 to 1955. The second period concerns the CPB’s interventions in public debates on unemployment and the growth of the public sector in the 1970s. Making use of two theoretical notions from sociology, policy device and public interventions, I will argue that the introduction of new macroeconomic models was crucial to the impact of the CPB’s interventions in both periods. Furthermore, I will argue that by targeting a broader audience in the 1970s, the CPB started to shift the mode of their expertise, from a facilitating role towards a watchdog role.