  The quark and gluon structure of the proton has been under intense
experimental and theoretical investigation for five decades. Even for the
distributions of the well-studied valence quarks, challenges such as the value
of the down quark to up quark ratio at high fractional momenta remain. Much of
the sea of quark-antiquark pairs emerges from the splitting of gluons and is
well described by perturbative evolution in quantum chromodynamics. However,
experiments confirm that there is a non-perturbative component to the sea that
is not well understood and hitherto has been difficult to calculate with ab
initio non-perturbative methods. This non-perturbative structure shows up,
perhaps most directly, in the flavor dependence of the sea antiquark
distributions. While some of the general trends can be reproduced by models,
there are features of the data that do not seem to be well described. This
article discusses the experimental situation, the status of calculations and
models, and the directions where these studies will progress in the near
future.
