Stanford University’s Protégé is the most widely used open source ontology editor and knowledge management toolset that can be downloaded from http://protege.stanford.edu. It supports reasoners such as HermiT and FaCT++ to validate ontologies for consistency, as well as a variety of other plugins. Originally developed as a Learning Health System for translating raw biomedical data into machine-readable data for decision-making, Protégé is now suitable for modelling, ontology-driven application development, and collaborative ontology engineering. The ontologies can be exported to many formats, such as RDFS and various OWL syntaxes.