Between 1940 and 1970, the population of the Toronto metropolitan region increased rapidly. This imposed new infrastructure demands, particularly for sewer, water supply, and transportation systems, and encouraged comprehensive approaches to planning and flood control. Several forms of expertise emerged to guide responses to these demands, of which three are considered here: engineering of urban services, planning of new communities, and watershed conservation. Each form of expertise had close ties to public- or private-sector institutions; collectively, they reinforced prevailing views concerning the public interest and the role of technocratic expertise. They also demonstrated how a city’s expert and political orders could be constitutive of each other, with the planning and building of infrastructure by government and the private sector creating the contexts for applying expertise, which, in turn, justified expansion of the city’s administrative functions.