This report develops a representation of focus of attention that circumscribes discourse contexts within a general representation of knowledge. Focus of attention is essential to any comprehension process because what and how a person understands is strongly influenced by where his attention is directed at a given moment. To formalize the notion of focus, the need for and the use of focus mechanisms are considered from the standpoint of building a computer system that can participate in a natural language dialogue with a user. Two ranges of focus, global and immediate, are investigated, and representations for incorporating them in a computer system as developed. The global focus in which an utterance is interpreted is determined by the total discourse and situational setting of the utterance. It influences what is talked about, how different concepts are introduced, and how concepts are referenced. To encode global focus computationally, a representation is developed that highlights those items that are relevant at a given place in a dialogue. The underlying knowledge representation is segmented into subunits, called focus spaces, that contain those items that are in the focus of attention of a dialogue participant during a particular part of the dialogue. Mechanisms are required for updating the focus representation, because, as a dialogue progresses, the objects and actions that are relevant to the conversation, and therefore in the participants? focus of attention, change. Procedures are described for deciding when and how to shift focus in task-oriented dialogues, i.e., in dialogues in which the participants are cooperating in a shared task. These procedures are guided by a representation of the task being performed. The ability to represent focus of attention in a language understanding system results in a new approach to an important problem in discourse comprehension?the identification of the referents of definite noun phrases. Procedures for identifying referents are developed that take discourse structure into account and use the distinction between highlighted items and those that are not highlighted to constrain the search for the referent of a definite noun phrase. Interpretation of an utterance also depends on the immediate focus established by the linguistic form of the preceding utterance. The interpretation of elliptical sentence fragments illustrates the effect of immediate focus. Procedures that interpret elliptical sentence fragments are developed. The use of a representation that superimposes syntactic information about an utterance on the interpretation of the underlying meaning of that utterance to minimize the processing required to expand a fragment into a complete sentence.