This report from The Young Foundation, commissioned jointly by the Homes and Communities Agency as part of the Future Communities programme, sets out a framework and online resource for built environment professionals and policymakers involved in planning, design, and creating communities and cities. It is based on an international review of new towns and communities and describes why some flourish and others fail. It finds that communities that do not work socially, at best fail to flourish, or at worst, spiral into decline. Critically, it finds that support services and interventions need to be designed at the right time for communities to function well in the long term, and provides practical advice about understanding how communities function socially.