PR-OWL - a language for defining probabilistic ontologies. Recent years have witnessed an increasingly mature body of research on the Semantic Web (SW), with new standards being developed and more complex problems being addressed. As complexity increases in SW applications, so does the need to cope with uncertainty. Several approaches to uncertainty representation and reasoning in the SW have emerged. Among these is Probabilistic Web Ontology Language (PR-OWL), which provides a means of representing uncertainty in ontologies expressed in Web Ontology Language (OWL). PR-OWL allows values of random variables to range over OWL datatypes, following an approach suggested by Poole et al. to formalizing the association between random variables from probabilistic theories with the individuals, classes and properties from ontological languages such as OWL.