Abstract: The impact of urban experimentation on urban planning approaches is so far insufficiently assessed and discussed. This thematic issue sets out to investigate the possibilities and limitations of ‘urban planning by experiment,’ defined as an approach that uses experimentation to innovate and improve urban planning instruments, approaches, and outcomes. It brings together eight contributions presenting original research on urban experimentation and its relation to urban planning. All contributions are empirically grounded in (illustrative) case studies, mostly from European cities. Here, we summarize and discuss the major findings across the eight contributions with respect to three key themes: the practices of urban experimentation, its outcomes, and its impacts on urban planning. We conclude that the practices of urban experimentation described in the contributions generated a wide variety of substantive and learning outcomes, which, according to the authors, represent worthwhile additions or alternatives to the current repertoire of approaches and instruments of urban planning. However, except for a single case, large-scale integration of experimentation in established approaches to urban planning was not observed, let alone a complete transformation of urban planning practices. An area for further research concerns the relation between the way urban experiments are organized and conducted, and their impact on urban planning.