  We provide an assessment of the impact of parton distributions on the
determination of LHC processes, and of the accuracy with which parton
distributions (PDFs) can be extracted from data, in particular from current and
forthcoming HERA experiments. We give an overview of reference LHC processes
and their associated PDF uncertainties, and study in detail W and Z production
at the LHC. We discuss the precision which may be obtained from the analysis of
existing HERA data, tests of consistency of HERA data from different
experiments, and the combination of these data. We determine further
improvements on PDFs which may be obtained from future HERA data (including
measurements of $F_L$), and from combining present and future HERA data with
present and future hadron collider data. We review the current status of
knowledge of higher (NNLO) QCD corrections to perturbative evolution and
deep-inelastic scattering, and provide reference results for their impact on
parton evolution, and we briefly examine non-perturbative models for parton
distributions. We discuss the state-of-the art in global parton fits, we assess
the impact on them of various kinds of data and of theoretical corrections, by
providing benchmarks of Alekhin and MRST parton distributions and a CTEQ
analysis of parton fit stability, and we briefly presents proposals for
alternative approaches to parton fitting. We summarize the status of large and
small x resummation, by providing estimates of the impact of large x
resummation on parton fits, and a comparison of different approaches to small x
resummation, for which we also discuss numerical techniques.
