Extensible and customizable: existing ontologies may be reused and customized Shareable: ontologies are designed for re-use Interoperable: ontologies can be shared between systems (because they are standards-based) Machine-readable: ontologies are expressed using URIs (or Uniform Resource Identifiers) for identifiers which can be resolved using any compatible system Amenable to inferencing: ontologies allow reasoning over the graph to discover new, implied relationships and make them explicit; this can be used for machine learning-type applications Ontologies essentially comprise two parts: the upper ontology, which is the schema used to define objects, their attributes, and relationships in a general way; and the lower (or domain) ontology, which is the data populating the schema(s).