  We report some recent progress towards classification of phenomenologically
appealing heterotic string models in the Free Fermionic Formulation. We focus
on a class of Z2xZ2 models with SO(10) space-time gauge symmetry and study
their main phenomenological aspects. We further consider reducing the gauge
symmetry to the Pati-Salam gauge group SU(4)XSU(2)_LxSU(2)_R and impose a
series of phenomenological constraints including the existence of gauge
symmetry breaking Higgs particles and the elimination of exotic fractionally
charged states.
