  A long-standing problem in string phenomenology has been the fact that the
string unification scale disagrees with the GUT scale obtained by extrapolating
low-energy data within the framework of the minimal supersymmetric standard
model (MSSM). In this paper we examine several effects that may modify the
minimal string predictions and thereby bring string-scale unification into
agreement with low-energy data. These include heavy string threshold
corrections, non-standard hypercharge normalizations, light SUSY thresholds,
intermediate gauge structure, and thresholds arising from extra matter beyond
the MSSM. We explicitly evaluate these contributions within a variety of
realistic free-fermionic string models, including the flipped SU(5), SO(6) x
SO(4), and various SU(3) x SU(2) x U(1) models, and find that most of these
sources do not substantially alter the minimal string predictions. Indeed, we
find that the only way to reconcile string unification with low-energy data is
through certain types of extra matter. Remarkably, however, many of the
realistic string models contain precisely this required matter in their
low-energy spectra.
