This special issue on intelligent user interfaces features a panorama of projects that highlight recent advances in the state of the art. Most of these projects have been presented at the premiere forum for intelligent user interface (IUI) research, the International Conference on Intelligent User Interfaces. The articles in this issue share three common themes. First, they describe projects that explore the boundaries of the man-machine interface. As a result, they examine issues that span both AI and human-computer interaction (HCI). Second, the articles all describe implemented systems. Although IUI work is evolving a strong theoretical basis, much of the cuttingedge activities take an “experimental systems” approach that revolves around iterative design, implementation, and evaluation of IUIs with human subjects. Third, the articles describe task-oriented IUIs, that is, IUIs designed to support a specific family of tasks such as planning or tutoring.