ASPEN (Advanced System for Process Engineering) is a software system for computer-aided process design. ASPEN was developed at M.I.T. during the period 1976-1981 under the sponsorship of the Department of Energy and 55 industrial participants. There are four manuals and one computer tape describing the maintenance, use, and installation of ASPEN. The ASPEN User Manual is a two volume manual describing how to use ASPEN. It is intended to serve both as a reference manual for users familiar with ASPEN and as a tutorial manual for the beginning user. The User Manual contains all the information needed to create an ASPEN Input File. It does not describe how to make an ASPEN run. Run procedures are computer system dependent and are described in the four ASPEN Installation Manuals. In addition to these manuals, over 100 publications, reports, and theses were produced by the ASPEN project. The ASPEN Project Final Report serves as a guide to these documents.