Making digital library content interoperable

The demand for powerful and rich Digital Libraries able to support a large variety of interdisciplinary activities has increased the need for "building by re-use" and sharing, especially when dealing with the content space. Interoperability is a central issue to satisfy these needs. Despite its importance, and the many attempts to address it done in the past, the solutions to this problem are today, however, still very limited. Main reasons for this slow progress are lack of any systematic approach for addressing the issue and scarce knowledge of the adopted solutions. Too often these remain confined to the systems they have been designed for. In order to overcome this lack, this paper proposes an Interoperability Framework for describing and analyzing interoperability problems and solutions related to use of content resources. It also discusses the many facets content interoperability has and provides a comprehensive and annotated portfolio of existing approaches and solutions to this challenging issue.