Data about public school facilities and a vision for equitable access to great school environments drives the work of the 21st Century School Fund (21CSF) and the Center for Cities and Schools (CC+S). In this work shop, participants had an opportunity to see how we link the data, analysis, technology tools and policy and practice reforms. Participants worked with one of the tools--the Joint Use Cost Calculator--and with data supporting this tool, and learn how it can affect policy and practice. The workshop begin with a short presentation on the theory of change used and tested over 20 years at the 21CSF; and used and tested at the CC+S over the last 10 years. Following this presentation; participants learned to work with the Joint use Cost Calculator and explored its use in advancing policy and practice change associated with community use of public school buildings and grounds.