This collection brings together several perspectives on language varieties defined according to their contexts of use—what are variously called registers, sublanguages, or genres. The volume highlights the importance of these central linguistic phenomena; it includes empirical analyses and linguistic descriptions, as well as explanations for existing patterns of variation and proposals for theoretical frameworks. The book treats languages in obsolescence and in their youth; it examines registers from languages from around the globe; and it offers several of the most complete studies of registers and register variation published to date, adopting both synchronic and diachronic perspectives.