  A summary is given of how spin enters in collinearly factorizing processes.
Next, theoretical aspects of polarization in processes beyond collinear
factorization are discussed in more detail, with special focus on recent
developments concerning the color gauge invariant definitions of transverse
momentum dependent distribution and fragmentation functions, such as the Sivers
and Collins effect functions. This has particular relevance for azimuthal
single spin asymmetries, which currently receive much theoretical and
experimental attention.
