Multimodal signal processing is an important research and development field that processes signals and combines information from a variety of modalities such as speech, vision, language, and text, which significantly enhance the understanding, modeling, and performance of human-computer interaction devices or systems enhancing human-human communication. The overarching theme of this book is the application of signal processing and statistical machine learning techniques to problems arising in this multi-disciplinary field. It describes the capabilities and limitations of current technologies, and discusses the technical challenges that must be overcome to develop efficient and user-friendly multimodal interactive systems.