This book is a cross-linguistic and interdisciplinary exploration of modality within systemic functional linguistics (SFL). Drawing upon the broad SFL notion of modality that refers to the intermediate degrees between the positive and negative poles, the individual papers probe into the modality systems in English and Japanese. The papers cover issues such as the conceptual nature of modality in both languages, the characterization of modulation in Japanese, the trans-grammatical aspects of modality in relation to mood and grammatical metaphor in both languages, and the modality uses and pragmatic impairment by individuals with a developmental disorder from a neurocognitive perspective. The book demonstrates a functional account of Japanese within an SFL model of language with a fresh perspective to Japanese linguistics. It also refers to cross-linguistic issues concerning how the principles and theories of SFL serve to empirically elaborate descriptions of individual languages, which will lead to the enrichment of the theory and practice of linguistics and beyond.