This PhD thesis explores the emergence of European environmental activism practices after 1968 and the subsequent assimilation of the ideas, participants, technologies, aesthetics, and design strategies thereof from the 1970s and in the early years of the new millennium, when the concept of sustainable urban development became normative in planning. Comprising five articles and a cover essay, the thesis is a critical historical analysis of sustainable urban development as a planning discourse, tool, and typology. Papers Ia and Ib frame environmentalism as insurgent planning practices with agency to transform normative planning from the margins. I propose the creation of a counter-archive of environmentalist zines to incorporate the stories and practices of these previously neglected actors, and as a means to reposition and expand the history of sustainable development, which is currently flat, simplified, or incomplete. Papers II and III explore intermediate stages in the assimilation of environmental activism practices through two case studies. Paper II analyses the 1976 exhibition ARARAT (Alternative Research in Architecture, Resources, Art, and Technology), in which alternative technologies and architectures focused on environmental protection were displayed at the Stockholm Museum of Modern Art and at the 37th Venice Biennale. Paper III studies the ecological, bottom-up community Understenshöjden, whose experiments with circular planning were later adopted by the housing company HSB to green their housing stock nationally. The final paper, Paper IV, tells the story of the 2001 international housing exhibition Bo01 in Malmö, which was designed to become a role model for urban sustainability. I explore how the entangled interests of public institutions, research, and manufacturing industries have shaped the now-institutionalized concept of sustainability and reflect on the ethics and design principles that sustainable practices manifest. This PhD study follows the shift of urban planning and capitalist urbanization from causing environmental degradation to becoming key agents for environmental sustainability and highlights the potential of planning history to critically narrate and contest contemporary techno-managerial and growth-oriented approaches to sustainability.