  The physical interpretation of generalized parton distributions (in the limit
$\xi=0$) as Fourier transforms of impact parameter dependent parton
distributions is discussed. Particular emphasis is put on the role of the
target polarization. For transversely polarized targets we expect a significant
deviation from axial symmetry for the distribution in the transverse plane. We
conjecture that this transverse distortion, in combination with the final state
provides a natural explanation for the sign of the Sivers contribution to
semi-inclusive single-spin asymmetries.
