  The focus of this talk is on the transverse components of parton momenta.
Like for collinear parton distribution functions (PDFs), we are also in the
case of transverse momentum dependent (TMD) PDFs, talking about forward matrix
elements. While the collinear PDFs describe only spin-spin correlations, the
TMD PDFs (or in short TMDs) include spin-momentum correlations, including also
time-reversal-odd (T-odd) correlations. The latter are important in the
description of single spin asymmetries. In this way TMDs open up new ways of
studying the spin structure of hadrons or they can be used as tools that
incorporate hadronic structure also in non-collinear situations. The operator
structure of TMDs within QCD, in particular the structure of Wilson lines, is
more complex than that for collinear functions leading to various ways of
breaking of universality. This breaking of universality, however, can be
handled within QCD.
