Multimodal Interfaces that enable natural means of interaction using multiple modalities such as touch, hand gestures, speech, and facial expressions represent a paradigm shift in humancomputer interfaces. Their aim is to allow rich and intuitive multimodal interaction similar to human-to-human communication and interaction. From the multimodal system’s perspective, apart from the various input modalities themselves, user context information such as states of attention and activity, and identities of interacting users can help greatly in improving the interaction experience. For example, when sensors such as cameras (webcams, depth sensors etc.) and microphones are always on and continuously capturing signals in their environment, user context information is very useful to distinguish genuine system-directed activity from ambient speech