Theoretical aspects of spin physics

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.