Over the last decades, desktop computers for professional and consumer applications have become a quasi standard. However, the last few years have been dominated by a new trend in computing: the mobile use of computers. Wearable computers, which are a special kind of mobile computer, assist their users in managing primarily physical tasks. Hendrik Witt examines user interfaces for wearable computers and analyses the challenges imposed by the wearable computing paradigm through its dual-task character. He discusses solutions for the development and evaluation of user interfaces which can be used in dual task scenarios. The author presents fundamental research results and introduces a special software tool as well as the "HotWire" evaluation method to facilitate user interface development and evaluation. Based on the findings of different end-user experiments conducted to study the management of interruptions with gesture and speech input in a wearable computing scenario, the author derives design guidelines and general constraints for forthcoming interface designs.