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.