This volume, the definitive reference manual for the latest version of MPI-1, contains a complete specification of the MPI Standard. Since its release in summer 1994, the Message Passing Interface (MPI) specification has become a standard for message-passing libraries for parallel computations. There exist more than a dozen implementations on a variety of computing platforms, from the IBM SP-2 supercomputer to PCs running Windows NT. The initial MPI Standard, known as MPI-1, has been modified over the last two years. This volume, the definitive reference manual for the latest version of MPI-1, contains a complete specification of the MPI Standard. It is annotated with comments that clarify complicated issues, including why certain design choices were made, how users are intended to use the interface, and how they should construct their version of MPI. The volume also provides many detailed, illustrative programming examples.