UrbanSim is an open source software platform for agent-based geospatial simulation, focusing on the spatial dynamics of urban development. Â Since its creation UrbanSim has been used in the official planningÂ processes for at least a dozen regional governments which were usedÂ to help allocate billions of dollars in regional investments in transportationÂ infrastructure.UrbanSim was first conceptualized in the late 1990′s and implementedÂ using the Java programming language. The technology landscape forÂ scientific computing changed dramatically after that, and by 2005Â UrbanSim was converted to Python, making heavy use of Numpy to vectorizeÂ calculations. By 2014, it became clear that UrbanSim should be reimplementedÂ again to take advantage of significant advances in the libraries availableÂ for scientific Python. The new version of UrbanSim, called UrbanSim2,Â makes extensive use of community-supported scientific Python librariesÂ to reduce the amount of domain-specific customized code to a minimum.UrbanSim is an excellent case study for the power of leveraging thework of the scientific programming community as scaffolding for adomain-specific application, as opposed to building an extensive customizedÂ solution in each domain. Additionally, the open and participatoryÂ nature inherent in nearly all of the open source projects describedÂ here has been particularly embraced by governments, who are oftenÂ reticent to support large commercial institutions and balkanized andÂ private data formats and software tools.