Space Syntax is an evidence-based approach to planning and design built on geospatial software. It focuses on the role of spatial networks in shaping patterns of social and economic transaction. Pioneered by Bill Hillier at University College London, Space Syntax is used by thousands of scholars worldwide in areas as diverse as archaeology, criminology and cognitive science as well as in its original discipline of architecture. Through the aegis of a consultancy company, Space Syntax Limited, the approach has, for over twenty years, been used by city municipalities and property developers to plan new development, most notably the redesign of Trafalgar Square in London. With commercial success behind them, the creators of Space Syntax now plan to “open source” the software. What does this mean and what are the reasons for doing so: philosophical, ethical or commercial? Who else is taking this route, what are the risks and what might be the future of urban planning software?