Information integration systems provide facilities that support access to heterogeneous information sources in a way that isolates users from differences in the formats, locations and facilities of those sources. A number of systems have been proposed that exploit knowledge based techniques to assist with information integration, but it is not always obvious how proposals differ from each other in their scope, in the quality of integration afforded, or in the cost of exploitation. This paper presents a framework for the comparison of proposals for information integration systems, and applies the framework to a range of representative proposals. It is shown that proposals differ greatly in all of the criteria stated and that the selection of an approach is thus highly dependent on the requirements of specific applications.