This paper evaluates the role of operational models in urban, physical planning. A theory of planning is proposed and used to guide discussion of the experience of urban development planning in the USA and UK during the last two decades. It is concluded that physical planning is an institution which controls the location and level of public investment in urban infrastructure and which produces plans of the development of urban areas as a means of increasing the profitability and reducing the risks associated with private land development. Operational models are used in this process to provide forecasts of development; the nature of these models can then be deduced.