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The report describes Ports, a unified method for communication between a computer program and terminals, files, peripheral devices, other programs, and supervisory software. In ISPL each job has a Port named MONITOR that handles resource allocation: creating and deleting files, assigning file space, core space, processor time. This design permits a hierarchical system of monitors, each controlling the jobs running under it. By routing output to a user terminal, Ports enable on-line debugging and simulation of rewritten files of programs. The Port concept improves modularity in 3 ways: Each connection need not be specified by the programmer but can be decided at execution; Linkage between programs is co-routine rather than subroutine, which simplifies programming, retains context, and removes the need for hierarchical organization; With different connections via Ports, the same system can be used in many ways, e.g., on-line or off, in simulation mode, audit-trailed, or data breakpointed.