AbstractThe 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 1015 heterotic string vacua with Pati–Salam, SU(4) × SU(2)L × 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.