This paper provides a comprehensive introduction to the emerging field of ontologies, focusing on their design and use. It highlights the importance of ontologies in improving communication, interoperability, and system engineering. The authors motivate the need for ontologies by discussing common problems such as conceptual and terminological confusion, which can lead to poor communication and interoperability issues in software systems. They outline a methodology for developing and evaluating ontologies, starting with informal techniques for scoping, handling ambiguity, and reaching agreement. The paper then delves into more formal approaches, emphasizing the role of formal languages and techniques in specification, implementation, and evaluation. Practical applications of ontologies are discussed through case studies, and the authors review existing software tools and techniques for ontology development. Finally, they address key research issues and future directions in the field.This paper provides a comprehensive introduction to the emerging field of ontologies, focusing on their design and use. It highlights the importance of ontologies in improving communication, interoperability, and system engineering. The authors motivate the need for ontologies by discussing common problems such as conceptual and terminological confusion, which can lead to poor communication and interoperability issues in software systems. They outline a methodology for developing and evaluating ontologies, starting with informal techniques for scoping, handling ambiguity, and reaching agreement. The paper then delves into more formal approaches, emphasizing the role of formal languages and techniques in specification, implementation, and evaluation. Practical applications of ontologies are discussed through case studies, and the authors review existing software tools and techniques for ontology development. Finally, they address key research issues and future directions in the field.