This project is an attempt to recuperate citizen participation in urban design and planning from the top-down attitudes that make them less and less relevant to informing the complex, dynamic ways that public space is actually produced. It critiques the status quo modes of practice and conceives of urban activism as reactions against that system, reactions that follow particular dispositions between preservation of the status quo to revolt. It focuses on two modalities of urban activism that hold promise for transformation--reform activism and autogestión. The dissertation specifically examines experiments in participatory urbanism in Barcelona, through which urban activists engage with institutions, organizations, and residents to either reform existing systems or to build alternative systems that are semi-autonomous (autogestionado). Using archival and ethnographic methods, this study offers insights on emerging activist roles that designers and urbanists are assuming in an effort to give citizens more local control over urban space. Understanding these new roles is important not only for aspiring urban activists but also to officials and other professionals who likewise must negotiate the dynamic terrain between institutions, professionals and publics.