The PDF4LHC Working Group provides an updated recommendation for the usage of parton distribution functions (PDFs) and the assessment of PDF and PDF+$\alpha_s$ uncertainties suitable for applications at the Large Hadron Collider (LHC) Run II. The group reviews recent developments in PDF determination, including new experimental measurements and methodological improvements. They compare the most recent PDF sets from various groups and propose a new prescription for combining a subset of available PDF sets into a single combined PDF set. The paper discusses tools for delivering this combined set in terms of optimized sets of Hessian eigenvectors or Monte Carlo replicas and provides examples of their application to LHC phenomenology. The PDF4LHC15 combined sets are presented, along with guidelines for their usage, formulae for calculating PDF and PDF+$\alpha_s$ uncertainties, and a citation policy. The paper also explores future directions for PDF4LHC activities.The PDF4LHC Working Group provides an updated recommendation for the usage of parton distribution functions (PDFs) and the assessment of PDF and PDF+$\alpha_s$ uncertainties suitable for applications at the Large Hadron Collider (LHC) Run II. The group reviews recent developments in PDF determination, including new experimental measurements and methodological improvements. They compare the most recent PDF sets from various groups and propose a new prescription for combining a subset of available PDF sets into a single combined PDF set. The paper discusses tools for delivering this combined set in terms of optimized sets of Hessian eigenvectors or Monte Carlo replicas and provides examples of their application to LHC phenomenology. The PDF4LHC15 combined sets are presented, along with guidelines for their usage, formulae for calculating PDF and PDF+$\alpha_s$ uncertainties, and a citation policy. The paper also explores future directions for PDF4LHC activities.