MPI-4.0 is a major update to the MPI standard. The largest changes are the addition of large-count versions of many routines to address the limitations of using an int or INTEGER for the count parameter, persistent collectives, partitioned communications, an alternative way to initialize MPI, application info assertions, and improvements to the definitions of error handling. In addition, there are a number of smaller improvements and corrections. Any valid MPI-3.1 program is a valid MPI-4.0 program with the exception of semantic changes listed in Chapter Semantic Changes and Warnings.