MAKING ENDS MEET - STRING UNIFICATION AND LOW-ENERGY DATA

We examine the effects that may resolve the disagreement between strin g-scale unification and low-energy data. ?These include string thresho ld corrections, nonstandard hypercharge normalizations. Light supersym metric thresholds, intermediate gauge structure, and extra matter beyo nd the minimal supersymmetric standard model. We explicitly evaluate t hese effects within the flipped SU(5), SO(6) X SO(3), and various SU(3 ) X SU(2) X U(1) string models, and surprisingly find that the only wa y to reconcile string unification with low-energy data is through cert ain types of extra matter. Remarkably, many of the realistic string mo dels contain precisely this required matter in their massless spectra.