Foreword This volume contains the papers presented at the 5th International Workshop on Uncer- tainty Reasoning for the Semantic Web (URSW 2009), held as a part of the 8th Inter- national Semantic Web Conference (ISWC 2009) at the Westfields Conference Center near Washington, DC, USA, October 26, 2009. It contains 6 technical papers and 3 po- sition papers, which were selected in a rigorous reviewing process, where each paper was reviewed by at least four program committee members. The International Semantic Web Conference is a major international forum for presenting visionary research on all aspects of the Semantic Web. The International Workshop on Uncertainty Reasoning for the Semantic Web is an exciting opportu- nity for collaboration and cross-fertilization between the uncertainty reasoning com- munity and the Semantic Web community. Effective methods for reasoning under un- certainty are vital for realizing many aspects of the Semantic Web vision, but the abil- ity of current-generation Web technology to handle uncertainty is extremely limited. Recently, there has been a groundswell of demand for uncertainty reasoning technol- ogy among Semantic Web researchers and developers. This surge of interest creates a unique opening to bring together two communities with a clear commonality of interest but little history of interaction. By capitalizing on this opportunity, URSW could spark dramatic progress toward realizing the Semantic Web vision. Audience: The intended audience for this workshop includes the following: (1) re- searchers in uncertainty reasoning technologies with interest in Semantic Web and Web- related technologies; (2) Semantic Web developers and researchers; (3) people in the knowledge representation community with interest in the Semantic Web; (4) ontology researchers and ontological engineers; (5) Web services researchers and developers with interest in the Semantic Web; and (6) developers of tools designed to support Semantic Web implementation, e.g., Jena, Prot´ eg´ e, and Prot´ eg´ e-OWL developers. Topics: We intended to have an open discussion on any topic relevant to the general subject of uncertainty in the Semantic Web (including fuzzy theory, probability the- ory, and other approaches). Therefore, the following list should be just an initial guide: (1) syntax and semantics for extensions to Semantic Web languages to enable repre- sentation of uncertainty; (2) logical formalisms to support uncertainty in Semantic Web languages; (3) probability theory as a means of assessing the likelihood that terms in different ontologies refer to the same or similar concepts; (4) architectures for applying plausible reasoning to the problem of ontology mapping; (5) using fuzzy approaches to deal with imprecise concepts within ontologies; (6) the concept of a probabilistic ontol- ogy and its relevance to the Semantic Web; (7) best practices for representing uncertain, incomplete, ambiguous, or controversial information in the Semantic Web; (8) the role of uncertainty as it relates to Web services; (9) interface protocols with support for uncertainty as a means to improve interoperability among Web services; (10) uncer- tainty reasoning techniques applied to trust issues in the Semantic Web; (11) existing implementations of uncertainty reasoning tools in the context of the Semantic Web; (12) issues and techniques for integrating tools for representing and reasoning with un- certainty; and (13) the future of uncertainty reasoning for the Semantic Web.