PSI is a structured, interpretive programming language designed to provide a programming system in which scientific calculation, graphics, and input/output relevant to academic problem solving can be performed quickly. It is a good language for the academic worker who must, from time to time, solve isolated problems or numerically verify new conjectures. The standard issue includes a complete user manual and scores of demonstration programs. Also included is Goliath, a programming environment similar to PSI but specially designed for arbitrary precision integers up to 700 digits.