This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book prepares the foundations of an approach to segregation in networks. It explores the spatial behaviour and forms of mobility of different social groups as networks of appropriation that in turn constitute possibilities of co-presence and absence. The book provides the city as one of the manifestations of the public sphere and an active means of exercising a 'politics of the everyday', the expression of different opinions and the possibilities for political organisation according to the spontaneous conditions of encounter. It also explores the relationship between space and social processes as a relationship between urban form and interaction – expressed as 'mutual effects'. The book discusses an approach that attempts to grasp traces of the elusive effects of built form and distinguish them from other urban forces at play, as a way of verifying their existence and extent.