The Population and Community Ontology (PCO) models material entities, qualities, and processes related to collections of interacting organisms such as populations and communities. It is taxon neutral, and can be used for any species, including humans. The classes in the PCO are useful for describing evolutionary processes, organismal interactions, and ecological experiments. Practical applications of the PCO include community health care, plant pathology, behavioral studies, sociology, and ecology. The PCO is complient with the Basic Formal Ontology (BFO) and is designed to be compatible with other OBO Foundry ontologies.