Mobile phones are not only a ubiquitous social accessory, but rapid technology advances have transformed them into feature-rich, Internet-enabled mobile PCs - a role once reserved for touchscreen-based personal digital assistants (PDAs). Although the most widespread phone styles in circulation feature the classic combination of numeric keypad and non-touchscreen display, larger touchscreen devices are gaining ground, as indicated by the fervor surrounding new devices such as Apple's iPhone and LG's Prada phone. Yet as devices evolve, users will remain constrained by the limits of their own visual, physical, and mental resources. My research has focused on the specific limitation that mobile users often have only one hand available to operate a device, which can be especially problematic for touchscreen-based devices, since they are frequently designed for two-handed stylus operation. Considering the growing volumes of data that small devices can now store and connect to, as well as the expanding cultural role of mobile phones, improving usability in mobile computing has potentially enormous implications for user productivity, satisfaction and even safety.

My own exploratory surveys have suggested that one-handed use of mobile devices is very common but that today's hardware and software designs do not support users in performing many tasks with only one hand. Motivated by these findings, the research goal of this dissertation is to contribute substantial knowledge in the form of empirically backed design guidelines and interaction techniques for improving one-handed usability and operation of mobile devices, with particular emphasis on those with touch-sensitive displays. The guidelines for one-handed mobile device design are the product of a series of studies conducted in pursuit of foundational knowledge in user behavior, preference, thumb capabilities and touchscreen-thumb interaction characteristics for single-handed device use. I also demonstrate the application of these guidelines through the development and evaluation of four applications. Two involve designs for navigating among programs, one provides an interface for searching large data sets, and the last offers a generalized mechanism for controlling arbitrary touchscreen interfaces with a thumb. Each of these applications explores a different one-handed interaction technique and offers perspective on its viability for one-handed device use.