  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.
