  We discuss the treatment of intrinsic transverse momenta in high energy
scattering processes. Within the field theoretical framework of QCD the process
is described in terms of correlators containing quark and gluon fields. The
correlators, parameterized in terms of distribution and fragmentation
functions, contain matrix elements of nonlocal field configurations requiring a
careful treatment to assure color gauge invariance. It leads to nontrivial
gauge links connecting the parton fields. For the transverse momentum dependent
correlators the gauge links give rise to time reversal odd phenomena, showing
up as single spin and azimuthal asymmetries. The gauge links, arising from
multi-gluon initial and final state interactions, depend on the color flow in
the process, challenging universality.
