: Clinical Pathways can be viewed as workflows, comprising an ordering of activities with associated execution constraints. Workflow models allow formal representation, analysis and execution of workflows in the Clinical Pathways. We present a semantic web-based approach where the domain knowledge and the workflow model are modeled separately as ontologies, while the Clinical Pathway and the associated workflows are modeled as the instantiations of these ontologies. Our workflow model is based on the UML Activity Diagrams and OWL-S service ontology, and the execution semantics are based on Place/Transition Petri Nets. We demonstrate our approach by capturing the workflow of the Prostate Cancer Care Pathway.