Multimodal systems have attained increased attention in recent years, which has made possible important
improvements in the technologies for recognition, processing, and generation of multimodal information.
However, there are still many issues related to multimodality which are not clear, for example, the
principles that make it possible to resemble human-human multimodal communication. This chapter
focuses on some of the most important challenges that researchers have recently envisioned for future
multimodal interfaces. It also describes current efforts to develop intelligent, adaptive, proactive, portable
and affective multimodal interfaces
