  T-duality is a symmetry of the heterotic string to all orders in string
perturbation theory. This results in an effective four dimensional supergravity
theory with desirable features for phenomenology. T-duality, as well as,
generically, an anomalous U(1), is broken by quantum anomalies of the effective
field theory. The structure of the full anomaly is presented, and the
mechanisms for anomaly cancellation are described.
