This contribution to the Italian "Workshop sui Monte Carlo, la Fisica e le
Simulazioni a LHC", held at LNF, Frascati, in February, May and October 2006,
summarises the status of parton density functions (PDF's) and the impact of
their uncertainties on the LHC physics. Emphasis is given to methods of
contraining PDF's using LHC data. Moreover, the advantages of the so-called PDF
reweighting technique, which enables to quickly estimate PDF uncertainties with
Monte Carlo events, are also presented.Comment: 9 pages, 5 figures, to appear in the proceedings of the Italian
"Workshop sui Monte Carlo, la Fisica e le Simulazioni a LHC", LNF, Frascati,
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