  We review the current state of the art in the determination of the parton
substructure of the nucleon, as expressed in terms of parton distribution
functions (PDFs), and probed in high-energy lepton-hadron and hadron-hadron
collisions, and we assess their implications for current precision collider
phenomenology, in particular at the Large Hadron Collider (LHC). We review the
theoretical foundations of PDF determination: the way cross sections are
expressed in terms of PDFs using perturbative QCD factorization and evolution,
the methodology used to extract PDFs from experimental data, and the way in
which different physical processes can be used to constrain different PDFs. We
summarize current knowledge of PDFs and the limitations in accuracy currently
entailed for the computation of hadron collider processes, in particular at the
LHC. We discuss the current main sources of theoretical and phenomenological
uncertainties, and the direction of progress towards their reduction in the
future.
