A hypermedia link service is system which stores the information describing hypertext links in a database which is separate from the data content over which the links are intended to operate. One of the first open hypermedia link services was Microcosm, which takes this philosophy to the extreme, storing not only the links in a separate database, but also the information about the endpoints of the links. The most important advantage of such an organisation is that the system remains open so that hypertext functionality may be extended to third party applications. The first part of this thesis describes the background to open hypermedia link services and describes the Microcosm system, which was developed by the Multimedia Research Group at the University of Southampton. The major problem with storing all the information about links separately from the content is that such a scheme introduces many opportunities for the introduction of inconsistencies and the loss of integrity of the hypermedia data model. The second part of this thesis examines these problems, and proposes a number of solutions. It concludes that no one solution can resolve all the problems, and that in order to ensure integrity it is necessary to impose some conditions which limit the degree of openness.