  The transverse nucleon spin can be transferred to the quarks and gluons in
several ways. In the factorizing, hard scattering processes to be considered,
these are parameterized at leading twist by the transversity distribution
function and at next-to-leading twist by quark-gluon correlation functions. The
latter enter the description of the structure function $g_2$ and possibly of
single spin asymmetries. It is discussed what is known about these functions
and what are the remaining open issues.
