LHAPDF is a general purpose C++ interpolator, used for evaluating PDFs from discretised data files. Previous versions of LHAPDF were written in Fortran 77/90 and are documented at http://lhapdf.hepforge.org/lhapdf5/. LHAPDF6 vastly reduces the memory overhead of the Fortran LHAPDF (from gigabytes to megabytes!), entirely removes restrictions on numbers of concurrent PDFs, allows access to single PDF members without needing to load whole sets, and separates a new standardised PDF data format from the code library so that new PDF sets may be created and released easier and faster. The C++ LHAPDF6 also permits arbitrary parton contents via the standard PDG ID code scheme, is computationally more efficient (particularly if only one or two flavours are required at each phase space point, as in PDF reweighting), and uses a flexible metadata system which fixes many fundamental metadata and concurrency bugs in LHAPDF5. Compatibility routines are provided as standard for existing C++ and Fortran codes using the LHAPDF5 and PDFLIB legacy interfaces, so you can keep using your existing codes. But the new interface is much more powerful and pleasant to work with, so we think you’ll want to switch once you’ve used it!None