The aim of this special issue is to address a conceptual and empirical gap in the existing literature on sustainable cities. Uniquely, it brings together questions about the "what" (in this case, the content and representation of urban policy) of research on urban sustainability with "how" (the social organisation of knowledge and action) through the results of collaborative and comparative work. The special issue contains curated contributions that draw upon the findings of a comparative international project funded by Mistra (Swedish Foundation for Strategic Environmental Research) through the Mistra Urban Futures Centre, based in Gothenburg. The project sought to bring together two questions that are usually treated as separate in existing research approaches. They are the "what" and "how" of sustainable cities’ debates. By working in partnership with local policy-makers, practitioners and universities, each local area entered into a process of collaborative design in order to examine assumptions, expectations, processes and the outcomes of knowledge co-production. This paper introduces the conceptual ideas behind this initiative and so provides a frame for the reader to situate the contributions. It then outlines those articles to draw connections between them and concludes with a short summary of what research and societal lessons can be learnt from the project.