Starting from existing spreadsheet software, like Lotus 1-2-3 R , Excel R , or Spreadsheet 2000 R , we propose a sequence of enhancements to fully integrate constraint-based reasoning, culminating in a system for reactive, graphical, mathematical constructions. This is driven by our view of constraints as the essence of (spreadsheet) computation, rather than as an add-on tool for expert users. We call this extended computational metaphor, spreadspaces.