  This is a pedagogical review article surveying the various approaches towards
understanding gauge coupling unification within string theory. As is well
known, one of the major problems confronting string phenomenology has been an
apparent discrepancy between the scale of gauge coupling unification predicted
within string theory, and the unification scale expected within the framework
of the Minimal Supersymmetric Standard Model (MSSM). In this article, I provide
an overview of the different approaches that have been taken in recent years
towards reconciling these two scales, and outline some of the major recent
developments in each. These approaches include
  1) string GUT models;
  2) higher affine levels and non-standard hypercharge normalizations;
  3) heavy string threshold corrections;
  4) light supersymmetric thresholds;
  5) effects from intermediate-scale gauge and matter structure beyond the
MSSM;
  6) strings without supersymmetry; and
  7) strings at strong coupling.
