The requirements of a user modeling component for an expert interface are analyzed, and the main points of a proposed approach to user modeling are stated. The authors focus on a knowledge-based system, called UM-tool, devoted to creating, maintaining, and using explicit user models within an expert interface. UM-tool supports a novel approach to user modeling, which is based both on the use of stereotypes and on a dynamic reclassification scheme. The architecture of the system is described, the organization and content of its knowledge bases are illustrated, and the modeling mechanisms utilized are presented in detail. An example of the use of UM-tool in the frame of the information-retrieval-natural-language-interface (IR-NLI II) expert interface devoted to supporting end users in accessing online information retrieval services is discussed, focusing on the specific role of the user modeling component. An evaluation of the proposed approach and a critical comparison with related work are presented. Future research directions are outlined.< >