Parton distribution functions of the proton

Parton distribution functions (pdfs) are non-perturbative inputs necessary to calculate cross-sections for scattering processes involving hadrons in the initial state. They are obtained by fitting theoretical predictions (based on a set of pdfs which are parameterized at a “starting scale”, and evolved to any scale via the DGLAP evolution equations) to various sets of experimental measurements. This paper presents an overview of our knowledge of parton distribution functions in the proton. Recent developments in QCD fits performed at next-to-leading order (NLO) and at next-to-next-to-leading order (NNLO) are emphasized.