A user interface software tool helps developers design and implement the user interface. Research on past tools has had enormous impact on today's developers—virtually all applications today are built using some form of user interface tool. In this article, we consider cases of both success and failure in past user interface tools. From these cases we extract a set of themes which can serve as lessons for future work. Using these themes, past tools can be characterized by what aspects of the user interface they addressed, their threshold and ceiling, what path of least resistance they offer, how predictable they are to use, and whether they addressed a target that became irrelevant. We believe the lessons of these past themes are particularly important now, because increasingly rapid technological changes are likely to significantly change user interfaces. We are at the dawn of an era where user interfaces are about to break out of the “desktop” box where they have been stuck for the past 15 years. The next millenium will open with an increasing diversity of user interface on an increasing diversity of computerized devices. These devices include hand-held personal digital assistants (PDAs), cell phones, pages, computerized pens, computerized notepads, and various kinds of desk and wall size-computers, as well as devices in everyday objects (such as mounted on refridgerators, or even embedded in truck tires). The increased connectivity of computers, initially evidenced by the World Wide Web, but spreading also with technologies such as personal-area networks, will also have a profound effect on the user interface to computers. Another important force will be recognition-based user interfaces, especially speech, and camera-based vision systems. Other changes we see are an increasing need for 3D and end-user customization, programming, and scripting. All of these changes will require significant support from the underlying user interface sofware tools.