Zoomable user interfaces (ZUIs) have received a significant amount of attention in the 18 years since they were introduced. They have enjoyed some success, and elements of ZUIs are widely used in computers today, although the grand vision of a zoomable desktop has not materialised. This paper describes the premise and promise of ZUIs along with their challenges. It describes design guidelines, and offers a cautionary tale about research and innovation. (Contains 2 figures, 2 tables, and 1 note.)