Abstract: "This paper describes a formalism for modular, incremental modelling of the beliefs and intentions of the human user of an intelligent interactive information system. The formalism makes use of partial models of a formal logical language with provisions for expressing epistemic and intentional attitudes, the choice of which is inspired by an analysis of the transfer of information in natural-language information-seeking dialogues. These models are designed to be incremental in the sense that the model grows as more information becomes available; this in contrast with standard logical models for expressing knowledge or belief. The formalism is modular in that information relating to different epistemic and intentional attitudes is contained in different 'modules', with the result that only certain modules have to be consulted in order to decide whether the model represents certain information, and that only certain modules have to be considered when information is to be stored in the model. The use of the model in dialogue management is briefly considered. The construction and update of a model on the basis of interpreting incoming communicative actions is described, and the generation of communicative acts on the basis of the model is outlined."