Digital documents often do not exist in isolation but are implicitly or explicitly linked to parts of other documents. The hyperlink concept, which was instrumental in the success of the World Wide Web, is considered the basis for creating and managing relations between documents. Using hyperlinks, most recent digital document formats as well as existing link systems enable users to associate information within as well as across different documents. However, due to a lack of empirical studies that investigate the end-user needs and behaviour when associating information within and across documents, the development of most existing document linking approaches is not driven by end-user requirements. Furthermore, existing document linking solutions often show some shortcomings in terms of the offered link granularity and cannot easily be extended to support new document formats. Most existing document formats only support hyperlinks to web resources but do not provide support for linking to parts of arbitrary third-party documents. In addition, the majority of current link systems enable the linking to a predefined set of document formats but it is not evident how the architecture of these systems can be extended to support new document formats.

In this dissertation we address the lack of user-driven and extensible cross-document linking solutions. Our approach consists of two major efforts including a user study and an architecture in combination with an extensible link service prototype. The user study relies on a multi-case design approach consisting of an online survey and interviews with participants of the online survey in order to investigate users' current behaviour in associating information as well as their appreciation and criticism of existing solutions. The insights from our user study enabled us to formulate a number of design implications for a dynamically extensible cross-document linking solution.

The presented cross-document link service, which is based on the RSL~hypermedia metamodel, meets end-user requirements and enables the linking of arbitrary documents at different levels of granularity. In our dynamically extensible cross-document link service, emerging document formats are supported via new data and visual plug-ins for the presented link browser or by integrating third-party document viewers via gateways. Our cross-document link service supports the dynamic extensibility and configuration of supported document formats without the need to redeploy the core link service. The presented link service currently supports the linking of six different document formats, with three of them being integrated with their third-party document viewers.

The extensibility of the presented link service has been verified in two different evaluations. An end-user study has further been conducted in order to evaluate the usability of the proposed cross-document link service. We feel confident that the presented concepts for a dynamically extensible cross-document link service improve the maintainability of documents in so-called cross-media information spaces and enable the future-proof linking across different document formats.