The Nonlinear Circuit Analysis Program NCAP is a circuit analysis code which uses the Volterra approach to solve for the transfer functions and node voltages of nonlinear circuits. To increase the transportability, the code was written in ANSI FORTRAN. The code was revised and documented in a joint effort using both in-house and contracted manpower. This documentation is a result of that effort. The documentation is made up of three volumes the Engineering Manual, the Users Manual, and the Programmers Manual. The Engineering Manual, Volume I, contains the introduction to the documentation, a description of the theory upon which NCAP is based, and the procedure for obtaining the input parameter data.