BioPortal is an open repository of biomedical ontologies, providing access via Web services and browsers to ontologies developed in OWL, RDF, OBO format, and Protégé frames. It offers functionalities for browsing, searching, and visualizing ontologies, as well as community-based participation through features like adding notes, mappings, and reviews. BioPortal also enables integrated search of biomedical data resources such as GEO, ClinicalTrials.gov, and ArrayExpress by annotating and indexing these resources with BioPortal ontologies. The repository includes a variety of ontologies covering subjects like anatomy, phenotype, and health, with over 134 ontologies and 680,000 total classes as of March 2008. BioPortal supports user contributions, peer reviews, and mappings between different ontologies, enhancing data integration and information retrieval. The platform's architecture is layered, using RESTful Web services for efficient data access and integration. BioPortal is open-source and domain-independent, making it a valuable resource for biomedical researchers, clinicians, and developers.BioPortal is an open repository of biomedical ontologies, providing access via Web services and browsers to ontologies developed in OWL, RDF, OBO format, and Protégé frames. It offers functionalities for browsing, searching, and visualizing ontologies, as well as community-based participation through features like adding notes, mappings, and reviews. BioPortal also enables integrated search of biomedical data resources such as GEO, ClinicalTrials.gov, and ArrayExpress by annotating and indexing these resources with BioPortal ontologies. The repository includes a variety of ontologies covering subjects like anatomy, phenotype, and health, with over 134 ontologies and 680,000 total classes as of March 2008. BioPortal supports user contributions, peer reviews, and mappings between different ontologies, enhancing data integration and information retrieval. The platform's architecture is layered, using RESTful Web services for efficient data access and integration. BioPortal is open-source and domain-independent, making it a valuable resource for biomedical researchers, clinicians, and developers.