This proposal explores a unified framework to solve Semantic Web tasks that often require similarity measures, such as RDF retrieval, ontology alignment, and semantic service matchmaking. Our aim is to see how far it is possible to integrate user-defined similarity functions (UDSF) into SPARQL to achieve good results for these tasks. We present some research questions, summarize the experimental work conducted so far, and present our research plan that focuses on the various challenges of similarity querying within the Semantic Web. 1 Motivation Semantic Web tasks such as ontology alignment, semantic service matchmaking, and similarity-based retrieval depend on some notion of similarity (at least if they are not solely based on logic). Therefore, researchers still try to find sound user-defined similarity functions (UDSF) to achieve good results for these tasks. Finding good similarity functions is, however, data- and context-dependent, and needs to be reconsidered every time new data ...