This book describes the subroutines provided by IBM's implementation of the Message Passing Interface (MPI) standard for Parallel Environment for AIX (PE). Programmers can use these subroutines when writing parallel applications. PE MPI intends to comply with the requirements of the Message Passing Interface Forum, MPI: A Message-Passing Interface Standard, Version 1.1, University of Tennessee, Knoxville, Tennessee, June 6, 1995 and MPI-2: Extensions to the Message-Passing Interface, University of Tennessee, Knoxville, Tennessee, July 18, 1997. PE MPI includes a partial implementation of MPI-2 at this time.