Given that the city is a system of relationships, design for city making means understanding, reinforcing and articulating this network. We posit plug-ins as situated design outcomes that aim to enrich the complex system of the city and expand its potentialities. Their role can be that of a generator (introducing new elements in a pre-existing urban context); a mediator (distilling existing practices into new landscapes); and an identifier (recognizing, visualizing and describing existing spaces and practices). This book’s central argument is that plug-ins are a solid yet supple conceptual framework for rethinking design’s agency in the city.