  Recent developments about the construction of standard $SO(10)$ and $SU(5)$
grand unified theories from 4-dimensional superstrings are presented. Explicit
techniques involving higher level affine Lie algebras, for obtaining such
stringGUTs from symmetric orbifolds are discussed. Special emphasis is put on
the different constraints and selection rules for model building in this string
framework, trying to disentangle those which are generic from those depending
on the orbifold construction proposed. Some phenomenological implications from
such constraints are briefly discussed.
