  Proton stability is one of the most perplexing puzzles in particle physics.
While the renormalizable Standard Model forbids proton decay mediating
operators due to accidental global symmetries, many of its extensions introduce
such dimension four, five and six operators. Furthermore, it is, in general,
expected that quantum gravity only respects local gauge, or discreet,
symmetries. String theory provides the arena to study particle physics in a
consistent framework of perturbative quantum gravity. An appealing proposition,
in this context, is that the dangerous operators are suppressed by an Abelian
gauge symmetry, which is broken near the TeV scale. A viable U(1) symmetry
should also be anomaly free, be family universal, and allow the generation of
fermion masses via the Higgs mechanism. We discuss such U(1) symmetries that
arise in quasi--realistic free fermionic heterotic--string derived models.
Ensuring that the U(1) symmetry is anomaly free at the low scale requires that
the Standard Model spectrum is augmented by additional states that are
compatible with the charge assignments in the string models. We construct such
string--inspired models and discuss some of their phenomenological
implications.

High Energy Physics - Phenomenology;High Energy Physics - Theory