  It is well-known that heterotic string compactifications have, in spite of
their conceptual simplicity and aesthetic appeal, a serious problem with
precision gauge coupling unification in the perturbative regime of string
theory. Using both a duality-based and a field-theoretic definition of the
boundary of the perturbative regime, we reevaluate the situation in a
quantitative manner. We conclude that the simplest and most promising
situations are those where some of the compactification radii are exceptionally
large, corresponding to highly anisotropic orbifold models. Thus, one is led to
consider constructions which are known to the effective field-theorist as
higher-dimensional or orbifold grand unified theories (orbifold GUTs). In
particular, if the discrete symmetry used to break the GUT group acts freely, a
non-local breaking in the larger compact dimensions can be realized, leading to
a precise gauge coupling unification as expected on the basis of the MSSM
particle spectrum. Furthermore, a somewhat more model dependent but
nevertheless very promising scenario arises if the GUT breaking is restricted
to certain singular points within the manifold spanned by the larger
compactification radii.
