Many interesting phenomena occur in material structures that are poised between rigid and flexible.In this talk,we describe the modern theory of rigidity and show how it can be used to analyze networks of constraints.These results can be used as input to geometrical simulation,where the various rigid parts of a system are moved,while maintaining all the constraints; both equalities and inequalities.This approach is applied to zeolites that are important for cracking petroleum and proteins where flexibility is often associated with function.