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 and provides many detailed, illustrative programming examples.