  Meson production data play an important role in our understanding of nucleon
structure. The combination of reaction channels is sensitive to gluon and
charge and flavor non-singlet quark densities and has the potential to provide
detailed images of the QCD quark structure of the nucleon. Quark imaging
requires a good understanding of the reaction mechanism, and in particular
rigorous tests of factorization of long- and short-distance physics. The higher
energies after the Jefferson Lab 12 GeV upgrade provide ideal conditions for
such studies, which are an essential prerequisite for studies of valence quark
distributions. An electron-ion collider would allow to extend these studies to
detailed imaging of sea quarks and gluons.
