This paper reviews the use of ontologies for integrating heterogeneous information sources, focusing on the role of ontologies, ontology representation, the use of mappings, and ontology engineering methods. The authors evaluate and compare various approaches, including SIMS, TSIMMIS, OBSERVER, CARNOT, Infoleuth, KRAFT, PICSEL, DWQ, Ontobroker, and SHOE. They discuss the three main architectures for using ontologies: single ontology, multiple ontology, and hybrid approaches. The paper also explores the additional roles of ontologies, such as query models and verification, and the use of mappings to connect ontologies with information sources and other ontologies. In the section on ontology engineering, the authors analyze the development methodologies, supporting tools, and the evolution of ontologies. The paper concludes by summarizing the state-of-the-art and identifying areas for further research, emphasizing the need for more sophisticated methodologies and better support for ontology development and evolution.This paper reviews the use of ontologies for integrating heterogeneous information sources, focusing on the role of ontologies, ontology representation, the use of mappings, and ontology engineering methods. The authors evaluate and compare various approaches, including SIMS, TSIMMIS, OBSERVER, CARNOT, Infoleuth, KRAFT, PICSEL, DWQ, Ontobroker, and SHOE. They discuss the three main architectures for using ontologies: single ontology, multiple ontology, and hybrid approaches. The paper also explores the additional roles of ontologies, such as query models and verification, and the use of mappings to connect ontologies with information sources and other ontologies. In the section on ontology engineering, the authors analyze the development methodologies, supporting tools, and the evolution of ontologies. The paper concludes by summarizing the state-of-the-art and identifying areas for further research, emphasizing the need for more sophisticated methodologies and better support for ontology development and evolution.