This book has attempted to give a complete description of the MPI specification, and includes code examples to illustrate aspects of the use of MPI. After reading the preceding chapters programmers should feel comfortable using MPI to develop message-passing applications. This final chapter addresses some important topics that either do not easily fit into the other chapters, or which are best dealt with after a good overall understanding of MPI has been gained. These topics are concerned more with the interpretation of the MPI specification, and the rationale behind some aspects of its design, rather than with semantics and syntax. Future extensions to MPI and the current status of MPI implementations will also be discussed.