PRESTO, an integrated system of computer codes, has been developed for the PREMPT program to perform EMP response analysis of communications systems. Volume One of the PRESTO User's Guide presents an overview of the entire PRESTO system and an introduction to the material contained in the other volumes (two through six) of the PRESTO User's Guide series. PRESTO provides engineering users with a tool for expeditiously analyzing electronic systems to determine electromagnetic pulse (EMP) effects. It is expected that users have a basic knowledge of digital computer usage, including FORTRAN. PRESTO consists of three groups of codes: an executive code, application codes, and a modeling library. The executive code provides the interface between the engineering user and the application programs which perform the calculations requested by the user. The modeling library contains subroutines used to model the electromagnetic response of signal cables, power systems, and ground systems. PRESTO interprets the user input, performs the required computation, and generates the output specified by the user. A modular approach is used in PRESTO to achieve maximum flexibility. The modularity of PRESTO and the independence of the executive control program facilitates the incorporation of additional application codes and modeling libraries applicable to system analyses other than for EMP effects.