To support the thriving multi-modal services, the cross-modal communication paradigm is proposed by the MultiMedia Communication (MMC) Research Group, which comprises researchers from Nanjing University of Posts and Telecommunications and Army Engineering University of PLA. This book offers a systematic and in-depth investigation into this highly promising research area. There are several specific concluding remarks given in the following. Firstly, this book provides a deep introduction to the characteristics of the current and future multi-modal services. Based on these characteristics, a general cross-modal communication architecture is constructed. Three key techniques, their encountered challenges, and design principles are also analyzed. It can be concluded that both the architecture construction and the key techniques design should center on the core notion of potential correlations among modalities or “semantic”. In other words, it should focus on uncovering, analyzing, and utilizing semantic information that reflects the potential correlations among modalities.