  Standard SUSY-GUTs such as those based on $SU(5)$ or $SO(10)$ lead to
predictions for the values of $\alpha _s$ and $sin^2\theta _W$ in amazing
agreement with experiment. In this article we investigate how these models may
be obtained from string theory, thus bringing them into the only known
consistent framework for quantum gravity. String models with matter in standard
GUT representations require the realization of affine Lie algebras at higher
levels. We start by describing some methods to build level $k=2$ orbifold
string models with gauge groups $SU(5)$ or $SO(10)$. We present several
examples and identify generic features of the type of models constructed.
Chiral fields appropriate to break the symmetry down to the standard model
generically appear in the massless spectrum. However, unlike in standard
SUSY-GUTs, they often behave as string moduli, i.e., they do not have
self-couplings. We also discuss briefly the doublet-triplet Higgs splitting. We
find that, in some models, built-in sliding-singlet type of couplings exist.
