An online library system is an example of a complex computer system in that it supports a variety of users, both patrons and staff, and is made up of many intricate programs with complex relationships among them. Certain features are essential to a user friendly system: (1) users cannot get lost in the system; (2) users cannot enter illegal commands; (3) users can choose among several simple modes of data entry; and (4) users are in control. These characteristics are exemplified by the Northwestern Online Total Information System (NOTIS), which is being installed at the University of Pennsylvania to form the heart of PennLIN, Penn's Library Information Network. Due to the complexity and multiplicity of functions of online catalogs, it has proven difficult to develop user interfaces that are adequate. Very little research has been done on the best format and content of screen displays. Preliminary findings indicate that graphic design and layout of displays are extremely important, but very few studies have focused on library applications. Opinion also varies on the use of color to enhance displays, and no definitive research results are known that support the superiority of color or monochrome displays for patron or staff use. Some conclusions based on observation and experience and questions that need research are presented, and it is concluded that although user interfaces have improved a great deal since the earliest attempts to design user-friendly systems, there is still a long way to go before online systems are truly user friendly. (MES)