  Discrete and global symmetries play an essential role in many extensions of
the Standard Model, for example, to preserve the proton lifetime, to prevent
flavor changing neutral currents, etc. An important question is how can such
symmetries survive in a theory of quantum gravity, like superstring theory. In
a specific string model I illustrate how local discrete symmetries may arise in
string models and play an important role in preventing fast proton decay and
flavor changing neutral currents. The local discrete symmetry arises due to the
breaking of the non--Abelian gauge symmetries by Wilson lines in the
superstring models and forbids, for example dimension five operators which
mediate rapid proton decay, to all orders of nonrenormalizable terms. In the
context of models of unification of the gauge and gravitational interactions,
it is precisely this type of local discrete symmetries that must be found in
order to insure that a given model is not in conflict with experimental
observations.
