The scope of this master´s thesis is the issue of usability in relation to interface design, in particular the interface of a university library´s online public accessed catalogue, or OPAC. Since the OPAC can be seen as the library´s ?shop window? to the world, it is of great importance that the interface to the OPAC can be used by patrons with effectiveness, efficiency and satisfaction. The master´s thesis is based on a qualitative study performed at Karolinska Institutets University library, and contains both a usability evaluation and an interview study. Two interfaces were evaluated: a standard web interface provided by the system supplier, and an interface developed in-house by Karolinska Institutet´s systems department. The two interfaces are both connected to the same online library catalogue, thus making possible a comparative study.