This paper focuses on the semantics of Telos, a language for representing knowledge about information systems. Telos is intended to support the development of information systems, especially in the requirements modeling phase. An object‐oriented representational framework is supported by Telos. Its features includeaggregation, generalization,andclassification,the treatment of attributes as first‐class objects and the explicit representation of time. Telos also provides an assertion sublanguage for expressing deductive rules and integrity constraints. A possible‐worlds semantics is defined for Telos knowledge bases. This semantics is intended to capture the peculiarities involved in the interpretation of temporal expressions. The integration of time has also inspired the treatment ofexistencein Telos. An ontology of objects based on the property of existence is proposed. In the spirit of KRYPTON, Telos knowledge bases are specified functionally, in terms of the operations provided for querying and updating them. This knowledge‐level analysis will allow us to specify exactly what a knowledge base can be ASK‐ed or TELL‐ed about the domain of discourse. Soundness, consistency, and completeness results have also been proven to complete the specification of Telos knowledge bases. This formal account of the language provides a logical framework that can be used to verify the correctness of any proposed implementation of