  The interpretation of LHC measurements requires a careful estimate of various
sources of uncertainties that affect theoretical calculations. In this
contribution, we present the PDF4LHC Working Group recommendations for the
usage of sets of parton distribution functions (PDFs) at the LHC Run II. We
review the construction and validation of the PDF4LHC15 combined sets, and
study some of their phenomenological implications. We also address some recent
criticism of these recommendations.
