  In this article we give a calculation of the two-loop $\sigma$-model
corrections to the T-duality map in string theory. We use the effective action
approach, and analyze two-loop corrections in a specific subtraction scheme.
Focusing on backgrounds which have a single Abelian isometry, we find the
explicit form for the $O(\alpha')$ modifications of the lowest order duality
transformations. Rather surprisingly, the manifest two-loop duality depends
crucially on the torsion field. In contrast to the dilaton and metric fields,
which are merely passive spectators, the torsion plays a more active role,
because of the anomalous couplings to the gauge fields that arise via
dimensional reduction. Our results support the interpretation of T-duality as
an expansion in the inverse string tension $\alpha'$, and its order-by-order
realization as a manifest symmetry of the full string theory.
