  This is intend to provide an overview of the theoretical and phenomenological
parts of the TMD (Transverse Momentum Dependent parton distribution and
fragmentation functions) studies. By comparing with the theoretical framework
that we have for the inclusive deep inelastic lepton-nucleon scattering and the
one-dimensional imaging of the nucleon, I try to outline what we need to do in
order to construct a comprehensive theoretical framework for semi-inclusive
reactions and the three dimensional imaging of the nucleon. After that, I try
to give an overview of what we have already achieved and make an outlook for
the future.
