In my thesis I will address the problem of interoperation between information spaces on the web. We explain how this problem is different to traditional database integration scenarios. In particular, we focus on one issue of the information integration problem peculiar to the web environment, namely linking information across sources. We use a graph-based data model and representation format, and define a query and rule language where queries can span multiple sources. We show how such a language can be used to relate information among sources.