The book is the result of research that started from the observation of how a process of rooting of informal cities is taking place within the contemporary global metropolises. This paradigm shift generates more and more processes of non-replacement, which are expressed through the physical body of the settlements. The book, therefore, aims to address the reading of this phenomenon in a systematic and rigorous way by analyzing the urban fabric of four favelas in Rio de Janeiro. The idea behind the work is to look at these territories with the eyes of the architect, overcoming the wall of unknowability represented by their complexity, marginalization, and difficulty of access. The aim is to build an unprecedented catalog of the informal, that collects the patrimony of actions, forms, and urban spaces generated within these settlements.