  The role of transverse momentum of quarks in semi-inclusive leptoproduction
will be discussed. It involves generalized distribution and fragmentation
functions which depend on both the longitudinal lightcone momentum fraction of
the quarks and on the transverse momentum. Constraints on these functions and
relations between them arise as a consequence of the QCD equation of motion for
the quark fields, Lorentz invariance and of C, P, and T invariance of the
strong interactions. Experimentally one has access to the functions for
instance in measurements of azimuthal asymmetries in hard processes.
