Understanding the 3-dimensional nucleon structure

The nucleons, neutrons and protons, form the almost totality of the visible matter in the Universe. They are composite objects, made of quarks and gluons, (partons) which interact according to the Quantum Chromo Dynamics rules. Despite many years of investigation, the full partonic 3-dimensional nucleon structure is still very mysterious. Do partons orbit inside the nucleon? How do their spin and orbital motion combine into the nucleon spin? How are they spatially distributed? What is the origin of the nucleon mass? All these questions have no clear answer, so far. Progress has been and is being made, both in theory and experiments, and much more is expected from future experiments. A summary of the situation, with emphasis on the momentum distribution of partons, is presented.