In the first years of its existence between 1957 and 1960 the efforts of the radical collective the Situationist International (SI) centred on its program of "unitary urbanism." This program sought to challenge the functionalist character of hegemonic forms of urban planning through novel practices of urban experimentation and contestation. Situationist urbanism arose largely through the collaboration between Guy Debord and the Dutch avant-garde architect Constant. This article explores the political dimension of situationist urbanism and the tensions that led to Constant’s secession from the group in 1960. Through analysis of the affinities and divergences between urbanism in its modernist and situationist forms a case is made for the crucial contribution situationist practices might make to the restoration of public space as a vital arena of contemporary political contestation and community.