The use of a voice interface; along with textual; graphical; video; tactile; and audio interfaces; can improve the experience of the user of a mobile device. Many applications can benefit from voice input and output on a mobile device; including applications that provide travel directions; weather information; restaurant and hotel reservations; appointments and reminders; voice mail; and e-mail. We have developed a prototype system for a mobile device that supports client-side; voice-enabled applications. In fact; the prototype supports multimodal interactions but; here; we focus on voice interaction. The prototype includes six voice-enabled applications and a program manager that manages the applications. In this chapter we describe the prototype; including design issues that we faced; and evaluation methods that we employed in developing a voice-enabled user interface for a mobile device.