This pioneering volume lays out a set of methodological principles to guide the description of interpersonal grammar in different languages. It compares interpersonal systems and structures across a range of world languages showing how discourse interpersonal relationships between the speakers and the purpose of their communication all play a role in shaping the grammatical structures used in interaction. Following an introduction setting out these principles each chapter focuses on a particular language - Khorchin Mongolian Mandarin Tagalog Pitjantjatjara Spanish Brazilian Portuguese British Sign Language and Scottish Gaelic – and explores mood polarity tagging vocation assessment and comment systems. The book provides a model for functional grammatical description that can be used to inform work on system and structure across languages as a foundation for functional language typology.