When two people talk, they focus their attention on only a small portion of what each of them knows or believes. Both what is said and how it is interpreted depend on a shared understanding of this narrowing of attention to a small highlighted portion of what is known. Focusing is an active process. As a dialogue progresses, the participants continually shift their focus and thus form an evolving context against which utterances are produced and understood. A speaker provides a hearer with clues of what to look at and how to look at it–what to focus on, how to focus on it, and how wide or narrow the focusing should be. As a result, one of the effects of understanding an utterance is that the listener becomes focused on certain entities (both objects and relationships) from a particular perspective. Focusing clues may be linguistic or they may come from knowledge about the relationships between entities in the domain. Linguistic clues may be either explicit, deriving directly from certain words, or implicit, deriving from sentential structure and from rhetorical relationships between sentences. This paper examines the relationships between focusing and definite descriptions in dialogue and its implications for natural language processing systems. It describes focusing mechanisms based on domain structure clues which have been included in a computer system and, from this perspective, indicates future research problems entailed in modeling the focusing process more generally.