Crime data are a record of incidents of crime. Such data are routinely compiled by police forces and feed into regional and national crime statistics. Crime data are relied upon in various narratives of crime in society. They are made selectively publicly available as open data and are also increasingly used in visualizations such as urban crime maps. Yet the nature of these data makes uncritical reliance upon them problematic. This paper explores the limits of crime data 21 the constraints on open data as crime data, and the deficiencies of visualizations of crime based on this data. It considers some of the practical, legal and institutional challenges of both expanding and improving these data.