  We review the current status of spin-averaged and spin-dependent parton
distribution functions (PDFs) of the nucleon. After presenting the formalism
used to fit PDFs in modern global data analyses, we discuss constraints placed
on the PDFs by specific data types. We give representative examples of
unpolarized and polarized PDFs and their errors, and list open questions in
global QCD fitting. Finally, we anticipate how future facilities, with
fixed-target and collider experiments, may impact our knowledge of PDFs and
reduce their uncertainties.
