I consider the uncertainties in parton distributions and the consequences for
hadronic cross-sections. There is ever-increasing sophistication in the
relationship between the uncertainties of the distributions and the errors on
the experimental data used to extract them. However, I demonstrate that this
uncertainty is frequently subsumed by that due to the choice of data used in
fits, and more surprisingly by the precise details of the theoretical framework
used. Variations in heavy flavour prescriptions provide striking examples.Comment: 10 pages, 15 figures as .ps or .eps files, invited talk at
PHYSTAT-LHC Workshop on Statistical Issues for LHC Physics, June 200
