In this introductory chapter to the book, we argue that the notion of complexity has not reached its full potential in reconceptualizing the field of language sciences. Others have argued that language use and language acquisition are in a mutually influencing relationship, but additional empirical evidence from a variety of authentic human interactions is needed. Indeed, the structural complexity of a language cannot be immediately matched to its learning complexity, or to the complexity of its use in context. We discuss alternative -- yet compatible -- complexity explanations for the self-organization of languages, the first pitting redundancy against economy, and the second focusing on environmental variables. The book examines the place of language in relation to interactive, pragmatic, multimodal discourse processes, but also in relation to cognition, argumentation and meaning-making, and to social structures and education. In doing so, our goal is to illustrate how complexity emerges as a network of functions which are organised differently and rooted in variable ways and which question each other