  Anomalous U(1) gauge symmetries can appear both in heterotic and type I
string theories. In the heterotic case we find a single anomalous U(1), while
in open string theories several such symmetries can appear. Nonetheless, there
is a conjectured duality symmetry that might connect these two theories. We
review the properties of anomalous gauge symmetries in various string theories
as well as the status of this heterotic-type I/II duality. We also comment on
the possible phenomenological applications of anomalous gauge symmetries in
string theory.
