Gauge Invariant Actions for String Models

String models of unified interactions are elegant sets of Feynman rules for the scattering of gravitons, gauge bosons, and a host of massive excitations. The purpose of these lectures is to describe progress towards a nonperturbative formulation of the theory. Such a formulation should make the geometrical meaning of string theory manifest and explain the many “miracles” exhibited by the string Feynman rules. As yet only partial success has been achieved in realizing this goal. Most of the material presented here already appears in the published literature but there are some new results on gauge invariant observables, on the cosmological constant, and on the symmetries of interacting string field theory.