  The nucleons (protons and neutrons) are by far the most abundant form of
matter in our visible Universe; they are composite particles made of quarks and
gluons, the fundamental quanta of Quantum Chromo Dynamics (QCD). The usual
interpretation of the nucleon dynamics in high energy interactions is often
limited to a simple one-dimensional picture of a fast moving nucleon as a
collection of co-linearly moving quarks and gluons (partons), interacting
accordingly to perturbative QCD rules. However, massive experimental evidence
shows that, in particular when transverse spin dependent observables are
involved, such a simple picture is not adequate. The intrinsic transverse
motion of partons has to be taken into account; this opens the way to a new,
truly 3-dimensional (3D) study of the nucleon structure. A review of the main
experimental data, their interpretation and understanding in terms of new
transverse momentum dependent partonic distributions, and the progress in
building a 3D imaging of the nucleon is presented.
