This thesis is a study of three public libraries, two in the Berlin district of Wedding, and one in Thornton Heath, south London. In these neighbourhoods with high levels of ethnic diversity, poverty and transience, the libraries offer a ‘window’ onto their localities, spaces in which local concerns, ideas and practices of contemporary multicultural urban life are played out. Through ethnographic fieldwork in two European cities, this thesis reflects on the particularity of the library as a local institution, and the ways in which larger political concerns emerge in these institutions. In interviews with library staff and in participatory work with library users, I trace how forms of social need and competency, questions of social difference and social justice, and pervasive concerns with demonstrations of value are spoken and unspoken in each site. In considering institutional narratives from library staff alongside the voices of library users, multiple interests and needs are made audible, and the library emerges as a space where expectations and priorities must be negotiated on a daily basis. The thesis explores the library as offering forms of public life and visibility to groups for whom ‘publicness’ is not a given: young children, older women, and teenagers. It argues for the library as an important interstitial space, a place ‘between’ the public life of the street and other forms of public participation, and as a site of social mediation. At the same time, it demonstrates the contingency of public space, the tensions around its use, and points where the library comes up against the limits of its institutional capacity. This thesis contributes to the sociology of public life, public space and public goods, exploring these issues through a highly visible yet under-researched institution, ‘placing’ this discussion within a nuanced account of the city neighbourhoods in which the research is located.