Planners are centrally concerned with the legitimacy of planning institutions and practices. In a democratic society, governments depend on the voluntary compliance of external actors for the implementation of their policies. Planning theorists have largely focused on the inclusiveness and quality of deliberation in goal-setting. This article expands this focus using Scharpf’s and Schmidt’s distinction between three domains of legitimation—input, throughput, and output—each of which affords a distinct pathway to legitimacy. These legitimation processes are examined through a comparison of the postwar development of American regional planning institutions in Minneapolis–St Paul, Minnesota, and Portland, Oregon. The input-throughput-output distinction can be used to interpret the operation and impacts of historical planning activities, or prospectively to evaluate the potential impacts of institutional reforms.