This lecture addresses this layered historical and contemporary knowledge of the city: How can we really see our built environment and understand its intertwinings that reveal and create genealogies? How can we organize its solutions in compendiums that preserve the theoretical principles and the tools used to design cities? And, finally, how do we extend this knowledge to contemporary urban projects, while avoiding imitation or replication? The case studies and examples of this talk will be European, but the methodology proposed can be applied to very different cultural contexts. The intention is to show how existing cities, examined with critical care, can be lessons for design itself – not limiting the creative possibilities of urban design today, but on the contrary giving it a solid disciplinary basis on which it can reinvent itself.