This paper describes the IFLA (International Federation of Library Associations and Institutions) guidelines for librarians charged with customizing OPAC (online public access catalog) software and vendors and producers of this software. The guidelines are intended to apply to all types of catalog, including World Wide Web-based catalogs, GUI (graphical user interface)-based interfaces, and Z39.50-Web interfaces. The focus is on the display of cataloging information (as opposed to circulation, serials check-in, fund accounting, acquisitions, or bindery information). The intent is to recommend a standard set of display defaults, defined as features that should be provided for users who have not selected other options, including users who want to begin searching without much instruction. The guidelines are divided into principles (general statements of the goals of the guidelines) and recommendations. The recommendations are organized based on the four main types of searches that users conduct in library catalogs: (1) for the works of a particular author or other creator; (2) for a particular work; (3) for works on a particular subject or in a particular form or genre; and (4) for works that take a particular disciplinary approach. Several controversies related to OPAC displays are also addressed. (MES)