Telos is a language designed to support the development of information systems by formally representing relevant knowledge. It is based on concepts from knowledge representation and is suitable for describing knowledge across various domains related to information systems, such as the subject world (application domain), usage world (user models, environments), system world (software requirements, design), and development world (teams, methodologies). The language features an object-centered framework for aggregation, generalization, and classification, a novel treatment of attributes, explicit time representation, and facilities for integrity constraints and deductive rules. It has been used in projects like LOKI and DAIDA, and its features include representing and reasoning about temporal knowledge, supporting linguistic extensions through metaattributes, and adapting concepts from deductive databases for query processing and integrity enforcement. Telos provides operations for constructing, querying, and updating structured knowledge bases. It supports temporal knowledge representation using a framework based on time intervals and relationships, allowing for the representation of incomplete information about time. The language also includes features for defining rules and constraints, such as integrity constraints and deductive rules, which are used to ensure data consistency and logical inference. Telos allows for the definition of metaclasses to introduce additional structure for particular categories of propositions, enabling the representation of complex relationships and constraints. The language is used to model knowledge about information systems across different subworlds, including the subject world (domain of the system), system world (specifications and implementations), usage world (environment and user interactions), and development world (design processes and teams). Telos supports the modeling of these subworlds through its classification dimension and treatment of attributes, allowing for the representation of diverse knowledge domains. The language has been applied in real-world scenarios, such as the ESPRIT project DAIDA, where it was used to build a complete information system development environment. Telos provides a flexible framework for representing and managing knowledge in information systems, supporting the development of complex systems through formal representation and reasoning.Telos is a language designed to support the development of information systems by formally representing relevant knowledge. It is based on concepts from knowledge representation and is suitable for describing knowledge across various domains related to information systems, such as the subject world (application domain), usage world (user models, environments), system world (software requirements, design), and development world (teams, methodologies). The language features an object-centered framework for aggregation, generalization, and classification, a novel treatment of attributes, explicit time representation, and facilities for integrity constraints and deductive rules. It has been used in projects like LOKI and DAIDA, and its features include representing and reasoning about temporal knowledge, supporting linguistic extensions through metaattributes, and adapting concepts from deductive databases for query processing and integrity enforcement. Telos provides operations for constructing, querying, and updating structured knowledge bases. It supports temporal knowledge representation using a framework based on time intervals and relationships, allowing for the representation of incomplete information about time. The language also includes features for defining rules and constraints, such as integrity constraints and deductive rules, which are used to ensure data consistency and logical inference. Telos allows for the definition of metaclasses to introduce additional structure for particular categories of propositions, enabling the representation of complex relationships and constraints. The language is used to model knowledge about information systems across different subworlds, including the subject world (domain of the system), system world (specifications and implementations), usage world (environment and user interactions), and development world (design processes and teams). Telos supports the modeling of these subworlds through its classification dimension and treatment of attributes, allowing for the representation of diverse knowledge domains. The language has been applied in real-world scenarios, such as the ESPRIT project DAIDA, where it was used to build a complete information system development environment. Telos provides a flexible framework for representing and managing knowledge in information systems, supporting the development of complex systems through formal representation and reasoning.