As a contribution to the history of Town and Country Planning this work shows how the Cambridge Preservation Society came to be instrumental in the formulation of a policy of containment for the city which led to the creation of the city's green belt. In doing so it describes how the Town Planning system developed in the 1930s and 1940s in relation to an historic town and how that system was manipulated by a private but influential group of individuals who had come together to preserve their local environment. The origins of the society are traced from the individuals who had campaigned for the drawing up of a regional plan to control indiscriminate building development on the fringes of the town (as it then was). The society was formed in 1928 with an Executive Committee drawn from leading members of Cambridge University and from the town. The work of the society is described, beginning with the purchase of open land on the outskirts of Cambridge to preserve it from building development and the purchase of restrictive covenants ("sterilisation") from the owners of other land nearby to the same end. The evolution of each of the milestones in the history of the planning of Cambridge and its hinterland is described, beginning with the Cambridgeshire Regional Planning Report (the "Davidge Report") of 1934 and including the pre-Second World War Cambridge & District Town Planning Scheme, the 1950 Holford Report on Cambridge and the first Cambridgeshire Development Plan of 1952. The society took part in each of these, consistently promoting the preservation of the character and setting of Cambridge as the 'only true university town in England.' The main aim of the society was achieved with the establishment of the Cambridge Green Belt, first proposed in 1958 but not formally approved until 1992. The development of that green belt over that period is described. The circumstances of the purchase by the society of the Wandlebury Estate on the summit of the Gog Magog Hills south east of Cambridge are described. The estate is now maintained by the society as a public country park.