It is generally acknowledged by the scientific community that ontologies may give a significant contribution to the design and implementation of better and more interoperable information systems, also in medicine. However, there is still much skepticism on the real impact that ontologies - apart from the academic world - may have on the design and maintenance of working information systems. The aim of this book is both to review fundamental theoretical issues on ontologies and to demonstrate the effectiveness of the ontological approach by illustrating real case studies. The first paper shows the usefulness of an ontological approach for solving some problems arising in medicine. It points out the relevance of terminological ontologies for disambiguating polysemous terms, for re-organizing very large corpora and detecting possible inconsistencies and for managing a catalog of bio-medical equipments. It also emphasizes the fundamental role played by ontologies when integration and interoperability of heterogeneous knowledge sources is needed, in particular in the field of clinical guidelines and evidence-based medicine. Other papers are a clear and relevant contribution to the topic of formal ontology in medicine: One introduces the application of the "descriptions and situations" theory in biomedicine. The others emphasize the important role of ontologies in modeling clinical guidelines: a crucial issue in healthcare management today. Two other interesting applications are in the field of genomics, the organ transplantation domain and where mistakes in medical ontologies come from and how they can be detected.