This paper aims to clarify the activities and techniques needed for building ontologies from scratch, presenting METHONTOLOGY, a structured methodology for ontology development. It highlights the differences between ontology development and knowledge-based system (KBS) development, emphasizing the need for standardized processes and techniques. The paper outlines the ontology development process, a life cycle for ontologies, and METHONTOLOGY as a method to build ontologies from scratch. The ontology development process involves several activities: planning, specification, knowledge acquisition, conceptualization, formalization, integration, implementation, evaluation, documentation, and maintenance. These activities are not necessarily sequential but are grouped into a life cycle that includes specification, conceptualization, formalization, integration, implementation, and maintenance. The evolving prototype life cycle is proposed as the most suitable for ontology development, allowing for flexibility and modification at any stage. METHONTOLOGY is a structured methodology for building ontologies from scratch, based on the authors' experience in developing an ontology in the chemical domain. It includes a set of activities, techniques, and deliverables for each phase of the ontology development process. The methodology emphasizes the importance of reusing existing ontologies, using formal languages for implementation, and ensuring the completeness, consistency, and conciseness of the ontology specification document. The paper also discusses the challenges of ontology development, including the need for documentation, evaluation, and maintenance. It highlights the importance of using intermediate representations during the conceptualization phase to ensure the accuracy and completeness of the ontology. The methodology proposes the development of an integration document to facilitate the reuse of existing ontologies and the inclusion of new definitions. The implementation phase requires the use of an environment that supports the selected meta-ontology and ontologies. Evaluation is a critical phase that ensures the correctness and completeness of the ontology, while documentation is an essential activity throughout the entire development process. The paper concludes that METHONTOLOGY provides a structured approach to ontology development, bridging the gap between ontological art and ontological engineering.This paper aims to clarify the activities and techniques needed for building ontologies from scratch, presenting METHONTOLOGY, a structured methodology for ontology development. It highlights the differences between ontology development and knowledge-based system (KBS) development, emphasizing the need for standardized processes and techniques. The paper outlines the ontology development process, a life cycle for ontologies, and METHONTOLOGY as a method to build ontologies from scratch. The ontology development process involves several activities: planning, specification, knowledge acquisition, conceptualization, formalization, integration, implementation, evaluation, documentation, and maintenance. These activities are not necessarily sequential but are grouped into a life cycle that includes specification, conceptualization, formalization, integration, implementation, and maintenance. The evolving prototype life cycle is proposed as the most suitable for ontology development, allowing for flexibility and modification at any stage. METHONTOLOGY is a structured methodology for building ontologies from scratch, based on the authors' experience in developing an ontology in the chemical domain. It includes a set of activities, techniques, and deliverables for each phase of the ontology development process. The methodology emphasizes the importance of reusing existing ontologies, using formal languages for implementation, and ensuring the completeness, consistency, and conciseness of the ontology specification document. The paper also discusses the challenges of ontology development, including the need for documentation, evaluation, and maintenance. It highlights the importance of using intermediate representations during the conceptualization phase to ensure the accuracy and completeness of the ontology. The methodology proposes the development of an integration document to facilitate the reuse of existing ontologies and the inclusion of new definitions. The implementation phase requires the use of an environment that supports the selected meta-ontology and ontologies. Evaluation is a critical phase that ensures the correctness and completeness of the ontology, while documentation is an essential activity throughout the entire development process. The paper concludes that METHONTOLOGY provides a structured approach to ontology development, bridging the gap between ontological art and ontological engineering.