This work is based on the observation that within contemporary metropolises there is a worldwide rooting process of informal cities. These informal cities are structured around themselves and thrive despite the odds; they settle in the collective imagination, thus producing an unprecedented social-cultural mutation. This paradigm shift is gradually triggering processes of “non-replacement” that can be observed in the physical pattern of the settlements, which begin to mutate and adapt to the logic of stabilization and reorganization. This work interprets this phenomenon systematically and thoroughly, analyzing the urban tissue of four favelas in Rio de Janeiro (a favorable context to observe informal cities) in morphological terms. We have spent much time traveling, observing, mapping, and drawing the territories, which allowed us to analyze them on a grand scale in order to explore the variations of the rooting process in the metropolis. It also helped analyze them on a small scale in order to identify and understand the syntax of their evolution and the micro-transformations of their urban tissue. The main aim was to examine these yet unexplored territories with the accuracy required for academic studies as well as observe them through the “eyes of the architect,” overcoming the difficulties represented by the hyper-complexity, marginalization and the difficulty of access. The aim was to create a new and precise catalog of the informal, which gathers examples of actions, forms and urban spaces created by the rooting process, and which can be used to better understand and interpret informal space.