We have designed, implemented, and empirically evaluated FARE, a functional realization system that exploits message specifications drawn from large-scale knowledge bases to create functional descriptions, which are expressions that encode both functional information (case assignment) and structural information (phrasal constituent embeddings). Given a message specification, FARE exploits lexical and grammatical annotations on knowledge base objects to construct functional descriptions, which are then converted to text by a surface generator. Two empirical studies---one with an explanation generator and one with a qualitative model builder---suggest that FARE is robust, efficient, expressive, and appropriate for a broad range of applications.