This paper concerns relationships among focus of attention, choice of referring expression, and perceived coherence of utterances within a discourse segment. It presents a framework and initial theory of centering intended to model the local component of attentional state. The paper examines interactions between local coherence and choice of referring expressions; it argues that differences in coherence correspond in part to the inference demands made by different types of referring expressions, given a particular attentional state. It demonstrates that the attentional state properties modeled by centering can account for these differences. PrefaceOur original paper (Grosz, Joshi, and Weinstein 1983) on centering claimed that certain entities mentioned in an utterance were more central than others and that this property imposed constraints on a speaker's use of different types of referring expressions. Centering was proposed as a model that accounted for this phenomenon. We argued that the coherence of discourse was affected by the compatibility between centering properties of an utterance and choice of referring expression. Subsequently, we revised and expanded the ideas presented therein. We defined various centering constructs and proposed two centering rules in terms of these constructs. A draft manuscript describing this elaborated centering framework and presenting some initial theoretical claims has been in wide circulation since 1986. This draft (Grosz, Joshi, and Weinstein 1986) has led to a number of papers by others on this topic and has been extensively cited, but has never been published. 1We have been urged to publish the more detailed description of the centering framework and theory proposed in Grosz, Joshi, and Weinstein (1986) so that an official version would be archivally available. The task of completing and revising this draft became more daunting as time passed and more and more papers appeared on centering. Many of these papers proposed extensions to or revisions of the theory and attempted to answer questions posed in Grosz, Joshi, and Weinstein (1986). It has become ever more clear that it would be useful to have a "definitive" statement of

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