This paper decribes a computational treatment of the semantics of relational nouns. It covers relational nouns such as "sister" and "commander", and focuses especially on a particular subcategory of them, called function nouns ("speed", "distance", "rating"). Relational nouns are usually viewed as either requiring non-compositional semantic interpretation, or causing an undesirable proliferation of syntactic rules. In contrast to this, we present a treatment which is both syntactically uniform and semantically compositional. The core ideas of this treatment are: (1) The recognition of different levels of semantic analysis; in particular, the distinction between an English-oriented and a domain-oriented level of meaning representation. (2) The analysis of relational nouns as denoting relation-extensions.The paper shows how this approach handles a variety of linguistic constructions involving relational nouns. The treatment presented here has been implemented in BBN's Spoken Language System, an experimental spoken language interface to a database/graphics system.