This paper describes how semantic knowledge technology can be used to enhance a digital library. The main research challenges in the field of digital librariesare examined and the extent to which semantic knowledge technology can be used to respond to these challenges is identified. Functionality developed within the SEKT project (http://sekt.semanticweb.org) is used to enhance an existing digital library. The development of a good ontology is central to such an application, and the particular ontology engineering approach adopted is described. Four broad challenges were identified: achieving interoperability; describing objects and repositories; managing multimedia collections; and improving user interfaces and human-computer interaction. The SEKT digital library case study is using semantic knowledge technology to respond to the first two and the last of these.