This volume presents a complete specification of the MPI-2 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. 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 MPI Forum, which has continued to work on MPI, has recently released MPI-2, a new definition that includes significant extensions, improvements, and clarifications. This volume presents a complete specification of the MPI-2 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.