This report is the PCL user's guide. It contains an overview of PICL and how it is used. Examples in C Fortran are included. PICL is a subroutine library that can be used to develop parallel programs that are portable across several distributed-memory multiprocessors. PICL provides a portable syntax for key communication primitives and related system calls. It also provides portable routines to perform certain widely-used, high-level communication operations, such as global broadcast and global summation. Finally, PICL provides execution tracing that can be used to monitor performance or to aid in debugging. 2 refs., 12 figs.