This introduction presents an overview of key concepts discussed in the subsequent chapters of this book. The book presents a typology of experimentation distinguishing between niche experiments, social experiments, transition experiments, grassroots experiments and sustainability experiments, and proposes an encompassing definition of experimentation. It highlights the connections between greening and social power that have significant political implications for urban experiments. Problems associated with climate change, economic under-development and social inequality are essentially urban in character. The ability of urban experiments to be radical in ambition while limited in scope underpins a vibrant debate in both the policy and academic worlds with respect to their ability to prompt genuine change. The process of urban experimentation unfolds over space and time through reworking the relationships between social and material networks in the context of existing economic, social and political trajectories. The book explores the relationships between social experiments, forms of expertise, and the emergence of a specifically urban mode of development.