Multimodal user interface (MUI) allows users to interact with a computer system through multiple human-computer communication channels or modalities. Users have the freedom to choose one or more modalities at the same time. MUI is especially important in mobile devices due to the limited display and keyboard size. In this chapter, we provide a survey of the MUI design in mobile technology with a speech-centric view based on our research and experience in this area (e.g., MapPointS and MiPad). In the context of several carefully chosen case studies, we discuss the main issues related to the speech-centric MUI in mobile devices, current solutions, and future directions.