This thesis explores structural changes to the institutions of urban life within the City of London during the fifteenth century. While the late medieval period posed many challenges, London fared well. Profound changes gripped its economic and social infrastructure: traditional medieval forms of social organisation and control changed into formalised structures and procedures, with implications for the social makeup of the City itself.    Using an innovative combination of methodologies, including GIS mapping and Social Network Analysis, social topography and sociability are investigated to reconstruct changes in ‘civil society’. Focusing upon four neighbouring parishes, the thesis is particularly concerned with personal interaction and locations of residence and trade. Archival study of hundreds of wills and deeds has created a dataset detailing legal and personal relationships between 4,000 Londoners. Social transformations are revealed at a local level by reconstructing and mapping property boundaries, and chronologies of ownership, as well as social relationships expressed in wills.     Early in the century the City was still segregated into relatively homogenous ‘trade quarters’, for both customary and pragmatic reasons. Bridge Street, for example, was a natural focus for Fishmongers. Prosopographical study of that Company has revealed a ‘quasi-federal’ structure, simultaneously reflecting neighbourhood identities and wider commercial interests. Yet, by the close of the century, a fundamental shift in the nature of Companies, from a ‘personal’, to a formal social basis, transformed the social topography of the City into a much more heterogeneous form.     The erosion of localised Craft structures coincided with the diversification of social activities of parish churches, revealed in the volume and breadth of community participation. Furthermore, the strength of informal sociability within the neighbourhood remained constant in the face of these changes. The neighbourhood thus remained a fundamental element in the infrastructure of the late-medieval City, both defining, and reflecting, local sociability