  The parton distribution functions (PDFs) are a non-negotiable input to almost
all theory predictions at hadron colliders. In this talk, I introduce PDF
determination by global analysis and discuss selected topics concerning recent
relevant data from HERA and the Tevatron, before giving some prospects for the
LHC. The combination of H1 and ZEUS cross sections reduces uncertainties and
will be an important input to future global PDF analyses. The theoretical
description of the heavy-quark contribution to structure functions at HERA has
a significant influence on predictions at the LHC. New W and Z data from the
Tevatron Run II provide important PDF constraints, but there are currently
problems describing the latest data on the lepton charge asymmetry from W -> l
nu decays. The Tevatron Run II jet production data prefer a smaller high-x
gluon than the previous Run I data, which impacts on predictions for Higgs
cross sections at the Tevatron. It is now possible to consistently calculate a
combined "PDF+alpha_S" uncertainty on hadronic cross sections, which is around
2-3% for the W and Z total cross sections at the LHC, reflecting their
potential as a "standard candle" to measure machine luminosity. Parton
luminosity functions are useful quantities for studying properties of hadronic
cross sections. Precision measurements at the LHC will provide further
constraints on PDFs as data accumulates in the early running period.
