This article describes the application GeoShare, which participated in the Semantic Web Challenge 2003. As their motivating example, the authors describe a tourist who is planning a bike trip and looking for nice locations for fishing and bathing. Therefore, he needs information regarding water quality-that is, information that has not only a thematic but also a spatial and temporal context. The authors provide a detailed look at the technologies GeoShare uses, including ontologies for describing vocabularies and catalogs as well as semantics-related keyword search. They also present their group's spatial reasoner and its application in a spatial data infrastructure consisting of a Web Catalog Service, distributed Web Map Services, and a Generic Web Map Client.