  A consistent phenomenological approach to the computation of transverse
single spin asymmetries in inclusive hadron production is presented, based on
the assumed generalization of the QCD factorization theorem to the case in
which quark intrinsic motion is taken into account. New k_T and spin dependent
quark distribution and fragmentation functions are considered: some of them are
fixed by fitting data on p(transv. polarized) + p -> pion + X and predictions
are given for single spin asymmetries in l + p(transv. polarized) -> pion + X
and gamma^* + p(transv. polarized) -> pion + X processes.
