This chapter reviews the main types of multimodal interfaces, their advantages and cognitive science underpinnings, primary features and architectural characteristics, and general research in the field of multimodal interaction and interface design. Multimodal systems have developed rapidly during the past decade, with steady progress toward building more general and robust systems, as well as more transparent human interfaces than ever before. Major developments have occurred in the hardware and software needed to support key component technologies incorporated within multimodal systems, as well as in techniques for integrating parallel input streams. Current natural language-processing algorithms typically rely heavily on the specification of determiners in definite and indefinite references in order to represent and resolve noun phrase reference. Over the past decade, numerous advantages of multimodal interface design have been documented. Unlike a traditional keyboard-and-mouse interface or a unimodal recognition-based interface, multimodal interfaces permit flexible use of input modes.