Transverse polarization distribution and fragmentation functions

We discuss transverse polarization distribution and fragmentation functions, in particular, T-odd functions with transverse momentum dependence, which might be relevant for the description of single transverse spin asymmetries. The role of intrinsic tranverse momentum in the expansion in inverse powers of the hard scale is elaborated upon. The sin(phi) single spin asymmetry in the process e \vec{p} -> e’ pi^+ X as recently reported by the HERMES Collaboration is investigated, in particular, by using the bag model.