This book develops a model of language which can be characterized as functionalist, usage-based, dynamic, and complex-adaptive. Its core idea is that linguistic structure is not stable and uniform, but continually refreshed and in fact reconstituted by the feedback-loop interaction of three components: usage, i.e. the interpersonal and cognitive activities of speakers in concrete communication; conventionalization, i.e. the social processes taking place in speech communities; and entrenchment, i.e. the cognitive processes taking place in the minds of individual speakers. Extending the so-called Entrenchment-and-Conventionalization Model, the book shows that what we call the Linguistic System is created, sustained, and continually adapted by the ongoing interaction between usage, conventionalization, and entrenchment. The model contributes to closing the gap in usage-based models concerning how exactly usage is transformed into collective and individual grammar and how these two grammars in turn feed back into usage. The book exploits and extends insights from an exceptionally wide range of fields, including usage-based cognitive linguistics, psycholinguistics, interactional linguistics and pragmatics, historical linguistics, sociolinguistics and the sociology and philosophy of language, as well as quantitative corpus linguistics. It makes numerous original suggestions about, among other things, how cognitive processing and representation are related and about the manifold ways in which individuals and communities contribute to shaping language and bringing about language variation and change. It presents a coherent account of the role of forces that are known to affect language structure, variation, and change, e.g. economy, efficiency, extravagance, embodiment, identity, social order, prestige, mobility, multilingualism, and language contact.