Ontology is back at the forefront of philosophy, science and technology. These days ontology comes in at least two main fashions: the traditional philosophical understanding of ontology has been recently flanked by a new – computer-based – understanding of ontology. The current resurgence of interest in ontological issues displays a number of novel features, both among philosophers and among information technologists.

Broadly speaking, the two research communities of philosophers and engineers have still not found a way to relate to each other systematically. One side is perhaps too theoretical, the other too pragmatic. However, in dynamic terms, one easily foresees mounting social and institutional pressure for the development of tools able to model fragments of reality in terms that are both adequate and efficient. The two volumes of TAO intend to play a role in paving the way for a better mutual understanding between engineers and philosophers. Since the two communities are still very different as to their own languages, conceptual tools and problem-sets, we devised two different volumes, one dedicated to the philosophical understanding of ontology and one to the computer-based understanding of ontologies. Both volumes contain both papers describing the state of the art in their respective topics and papers addressing forefront, innovative and possibly controversial topics.