An ontology is a computational representation of a domain of knowledgebased upon a controlled, standardized vocabulary for describing entities and the semantic relationships between them. The Human Phenotype Ontology (HPO) aims to provide a standardized vocabulary of phenotypic abnormalities encountered in human disease. Terms in the HPO describes a phenotypic abnormality, such as atrial septal defect. The HPO was initially developed using information from Online Mendelian Inheritance in Man (OMIM), which is a hugely important data resource in the field of human genetics and beyond. The HPO is currently being developed using information from OMIM and the medical literature and contains approximately 10,000 terms. Over 50,000 annotations to hereditary diseases are available for download or can be browsed using the PhenExplorer.The HPO project encourages input from the medical and genetics community with regards to the ontology itself and to clinical annotations.