  The main aspects of a recently developed method for classification of
heterotic superstring vacua in the Free Fermionic Formulation are presented.
Using these techniques we classify a big number of approximately $10^{15}$
heterotic string vacua with Pati--Salam, SU(4)x{SU(2)}_Lx{SU(2)}_R gauge
symmetry with respect to their main phenomenological features as the number of
families, Pati--Salam breaking Higgs, Standard Model Higgs doublets, additional
triplets and exotic charge states. We identify an interesting subclass of these
vacua, approximately one to one million, whose massless spectrum is completely
free of fractionally charge states.
