  The quark orbital angular momentum (OAM) has been recognized as an important
piece of the proton spin puzzle. A lot of effort has been invested in trying to
extract it quantitatively from the generalized parton distributions (GPDs) and
the transverse-momentum dependent parton distributions (TMDs), which are
accessed in high-energy processes and provide three-dimensional pictures of the
nucleon. Recently, we have shown that it is more natural to access the quark
OAM from the phase-space or Wigner distributions. We discuss the concept of
Wigner distributions in the context of quantum field theory and show how they
are related to the GPDs and the TMDs. We summarize the different definitions
discussed in the literature for the quark OAM and show how they can in
principle be extracted from the Wigner distributions.
