  The current status of global QCD analysis of parton distribution functions of
the nucleon is reviewed. Recent progress made in determining various features
of the parton structure of the nucleon, as well as outstanding open questions
are discussed. These include: the small-$x$ and large-$x$ behavior of the
partons, particularly the gluon; the differentiation of $u$ and $d$ quarks; the
strangeness sea ($s+\bar{s}$), the strangeness asymmetry ($s-\bar{s}$); and the
heavy quark distributions $c$ and $b$. Important issues about assessing the
uncertainties of parton distributions and their physical predictions are
considered. These developments are all critical for the physics programs of
HERA II, Tevatron Run II, RHIC, and LHC.
