  We present a short overview on the studies of transverse momentum dependent
parton distribution functions of the nucleon. The aim of such studies is to
provide a three dimensional imagining of the nucleon and a comprehensive
description of semi-inclusive high energy reactions. 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, we
summarize what we need to do in order to construct such a comprehensive
theoretical framework for semi-inclusive processes in terms of three
dimensional gauge invariant parton distributions. After that, we present an
overview of what we have already achieved with emphasize on the theoretical
framework for semi-inclusive reactions in leading order perturbative QCD but
with leading and higher twist contributions. We summarize in particular the
results for the differential cross section and the azimuthal spin asymmetries
in terms of the gauge invariant transverse momentum dependent parton
distribution functions. We also briefly summarize the available experimental
results on semi-inclusive reactions and parameterizations of transverse
momentum dependent parton distributions extracted from them and make an outlook
for the future studies.
