Ontology, in its philosophical meaning, is the discipline investigating the structure of reality. Its findings can be relevant to knowledge organization, as well as models of knowledge can in turn offer relevant ontological suggestions. Several philosophers in time have pointed out that reality is structured into a series of integrative levels, like the physical, the biological, the mental, and the cultural one, and that each level plays as a base for the emergence of more complex ones. Among them, more detailed theories of levels have been developed by Nicolai Hartmann and James K. Feibleman, and these have been considered as a source for structuring principles in bibliographic classification by both the Classification Research Group (CRG) and Ingetraut Dahlberg. CRG's analysis of levels and of their possible application to a new general classification scheme based on phenomena instead of disciplines, as it was formulated by Derek Austin in 1969, is examined in detail. Both benefits and open problems in applying integrative levels to bibliographic classification are pointed out.