Ontologies are increasingly used in the semantic description of biological and medical data from creation to publication and consumption by semantically enabled applications. To be effective, such ontologies must be of high quality and work well together. Therefore, issues like techniques for ontology use, good ontology design, ontology maintenance, and ontology coordination need to be addressed. The International Conference on Biomedical Ontology (ICBO) series is designed to meet this need. ICBO 2015, the sixth event in the highly successful series, will bring together representatives of all major communities involved in ontology use and development in biomedical research, health care, and related areas.