A national study of users and non-users of public access online catalog systems (OPACs) was undertaken at 29 libraries, representing 15 computer systems, to provide data and interpretations useful to system designers and library administrators. The findings indicate that users have overwhelmingly positive responses to OPACs. Non-users have positive attitudes toward OPACs, believe they are likely to use them eventually, and that they can learn to use them quickly and easily. Problem areas identified were using and understanding codes and abbreviations, understanding terminal displays, command and control, subject searching, known-item searching, patience with response time, queuing and terminal dialog, and library support services. The most important determinant of user satisfaction was effective subject searching, followed by library support services, command and control, and known-item searching. Users recommend that library administrators make terminals highly visible, provide printers, add terminals, expand databases, and improve systems rather than provide user training. Designers are asked to consider the user interface and to enhance subject searching, incorporate circulation information, and provide brief bibliographic display formats. This extensive report includes 12 appendices, 82 tables and 12 figures. (Author/LMM)