Quark and Gluon Orbital Angular Momentum: Where Are We?

The orbital angular momentum of quarks and gluons contributes significantly to the proton spin budget and attracted a lot of attention in the recent years, both theoretically and experimentally. We summarize the various definitions of parton orbital angular momentum together with their relations with parton distributions functions. In particular, we highlight current theoretical puzzles and give some prospects.
Comment: 6 pages, contribution to the proceedings of the LigthCone 2015 Conference, Sep 21-25, INFN Frascati National Laboratories, Frascati, Italy