### TMD Theory Overview.

P. J. Mulders
Transverse momentum dependent (TMD) distribution and fragmentation functions are described as Fourier transforms of matrix elements containing non-local combinations of quark and gluon fields. The x and $p_T$ dependent TMD functions appear in the parametrization of light-front correlators including a transverse (space-like) non-locality. The TMD functions relevant at leading order include spin-spin densities as well as momentum-spin densities and they are able to describe single-spin and azimuthal asymmetries, such as Sivers and Collins effects in...