ar X iv : h ep - p h / 94 11 21 8 v 1 3 N ov 1 99 4 CPTH - C 335 . 1094 NUB – 3109 hep - ph / 9411218 November 1994

We give a model-independent discussion of fermion masses in four-dimensional het-erotic superstring theories. We discuss the tree level contributions and quantum corrections, including one-loop threshold effects and masses generated as a result of non-perturbative supersymmetry breaking. We also point out that superstring models give rise to a generic µ-term in the effective low energy Lagrangian. The basic assumption of superstring phenomenology is that all light particles originate from superstring excitations that are massless at the superstring unification scale. The hierarchy between this high energy scale and small masses is created by the vacuum expectations of Higgs fields and by the supersymmetry breaking scale. Quarks and leptons acquire masses as a result of direct Yukawa couplings to Higgs scalars. The masses of squarks and sleptons receive contributions from the so-called soft terms generated by supersymmetry breaking. What is less known, and will be discussed later in this review, is that once local supersymmetry is broken, additional supersymmetric mass terms can also be generated for bosons and fermions, including the two Higgsinos of the minimal supersymmetric standard model (MSSM) and other fermions.