The multimodal information can be assimilated at three levels 1) early
fusion, 2) intermediate fusion, and 3) late fusion. Early fusion can be performed at the
sensor or signal level. Intermediate fusion can be at the feature level, and late fusion
may be done at the decision level. Apart from that, some more fusion techniques are
rank-based, adaptive, etc. This chapter provides an extensive review of studies based
on fusion and reported noticeable work herewith. Eventually, we discussed the
challenges associated with multimodal fusion.