This book looks to the moment of encounter between architectural design and informal settlements as the most extreme demonstration of an increasingly evident disciplinary fascination for urban informality. It is an enduring fascination, arising from the need to test the boundaries of the discipline in the hope of finding it adaptable to change and willing to adapt. It is also a fascination that feeds off the gap that exists between the search for a renewed relevance of disciplinary tools, and the wider loss of faith in the project as a way to envision societal change. In fact, such fascination is played out within a seemingly structural contradiction: informal settlements originate as the effect of economic and political strategies that are deployed at the global scale; conversely, when dealing with informality, architecture searches for legitimization at the very small scale of the tactical and ultralocal. A relationship of inverse proportion is in place, between the constrained scope of architectural design and the scale of the issues it sets out to address.