Building on theories of the social construction of technological systems, and the larger science and technology studies (STS) literature, this chapter explores the visions of urban sustainability to connect its multiple, shifting understandings and complexities. At the same time, this process of deconstruction and reconstruction opens up areas of political negotiation for further investigation and contestation. The chapter argues that this can only be done by politicizing sustainability, embracing and exploring the political nature of sustainability discourses. It discusses how power operates across sociotechnical systems, opening-up and closing-down potential pathways for urban transitions. The chapter examines the politics of scientific and technological knowledge and expertise, and the degree to which it has come to define urban sustainability discourse. It considers the importance of political dynamics in public engagement and deliberation. The chapter shows how power is unevenly distributed in sociotechnical systems.