We present a survey of recent research studies of the implementation of adaptive user models in human-computer interaction. A classification of research directions on adaptive user interfaces is first proposed; it takes account of the user characteristics that are modelled, the distribution of initiative and control of the system evolution between user and system, and the role of dynamic adaptation. Then, a few representative research studies are briefly presented to illustrate this classification. In the conclusion, some major issues regarding the utility and usability of adaptive user interfaces and the design of an appropriate methodology for assessing the ergonomic quality of this new form of interaction are mentioned.