Never before have cities been so important. Today, cities are home to the majority of the world’s population and production, and are the goal of millions of migrants around the world. They are complex, evolving places, where different modes of operating and experiencing life collide and converge. Yet, increasingly, our cities are growing informally, planned and built by non-professionals. Informality resembles an evolutionary process more than a simple absence of rules. In itself, informality is not illegal, dysfunctional, or indicative of poverty; in fact, its actors, skills and capital are probably our best chance to solve the world’s growing housing crises. We assume that informal is the daily condition of at least one sixth of our world’s population. It would be a waste to ignore collective knowledge created by a billion experts. Today, formal often stands for global, while informal might offer a local, alternative point of view. Architects must redefine their role in shaping cities and learn from this emerging intelligence. Lessons of Informality wants to uncover such potentials. Both parts of the title, “lessons” and “informality”, need to be understood broadly. The informal is more than its common physical representation, the images of slums and informal settlements that we quickly associate with the word. And lessons cannot be recipes or agendas, but rather must be concepts to absorb and adapt to site-specific solutions. Neither term can be idealized or generalized: a romantic view of informal settlements is dangerous and neglects the inhuman living conditions associated with some of these areas, while tabula rasa strategies destroy the economic and social backbone of displaced residents. And although many aspects and conditions are similar, no city or district is the same. Keeping this in mind, the book uses Addis Ababa and Ethiopia as examples to learn from informal processes and tries to point to whether and how they can be the basis for formal city planning. Concepts of multiplicity and locality naturally also apply to the content of this book. Each contributor defines informality in his or her own slightly different way, yet aims to apply a common understanding of its values to the historic, economic, social and spatial aspects of building and city planning. In film and writing, this variety of subjective stories reflects reality and hopefully stimulates the reader to imagine a critical dialogue between contributions, in order to find his or her own “objective” truth. By providing arguments for a new, creative, local and diverse framework for organizing and building in emerging territories, Lessons of Informality intends to raise awareness of a previously discredited wealth of information in city planning and management. It is from here that we begin to redefine the role of architecture and urban planning in today’s city making.