This book explores the topic of proximity and its relations in the design of contemporary urban fabrics and public spaces. The effects of the COVID-19 pandemic and reflections on the future of cities have lately shed light on the concept of proximity, which is intended as the relationship between communities and urban functions and as relations among people, built spaces, and open spaces. The proximity is a historic and fertile field of interest for American and Northern European urban studies; it is a spatial and social program seemingly surpassed by the styles and rhythms of contemporary city life, but today it is back in vogue with different purposes. Meanwhile, the action research developed by the Department of Architecture and Urban Studies at the Politecnico di Milano for the Municipality of Milan reached its conclusion (2018-2020). The research work focused on contextualizing the new M4 Metro line stations under construction, and jointed mobility flows and places, long-range networks and local ones, boosting the idea of metro stations as regenerative urban thresholds and urban platforms for enabling environmental, sustainable settlement, and active mobility systems. In other words, the action research for Milan shows how to achieve the concept of proximity in the urban design practice in a dense, stratified, and complex urban context.