Parton Distribution Functions and QCD Global Fitting

High energy hadrons interact through their quark and gluon constituents. The interactions become weak at short distances thanks to the asymptotic freedom property of QCD, which allows perturbation theory to be applied to a rich variety of experiments. The nonperturbative nature of the proton for single hard interactions is thus characterized by Parton Distribution Functions (PDFs) fa(Q, x) of momentum scale Q and light-cone momentum fraction x. The evolution in Q is determined perturbatively by QCD renormalization group equations, so the nonperturbative physics can be characterized by functions of x alone at a fixed small Q0. The ongoing project to extract those functions from experiment, and to estimate their uncertainties, is the subject of this talk. The applicability of single nucleon PDFs to hard scatterings between heavy nuclei is a key question to be addressed in the workshop.