This book for the first time reconstructs in a single theoretical framework the more important approaches to linguistic variation found in areas as different as historical linguistics, dialectology, sociolinguistics, psycholinguistics, stylistics, contrastive linguistics, language typology, so-called evaluation grammar, and current Chomskyan generative grammar (generally with an emphasis on syntax). The book concentrates on language-internal variation but also analyses typological research and considers the question of how linguistic descriptions may account for variation both within and between languages. The book's first and primary aim is adequate conceptualization in the area of linguistic variation. Its second aim is a practical one: to contribute, from a theoretical point of view, to the vast descriptive effort that is demanded in linguistics in documenting endangered languages. Its third aim is, simply, orientation. Using a non-Labovian notion of linguistic variable, the author distinguishes a holistic and a component approach to linguistic variation. A precise version of the former is developed by formulating a theory of language varieties based on the concept of variety structure of a language; it is then shown how the proposals made by major representatives of the component approach can be integrated into this framework. The theory is extended to interlanguage variation and applied, in particular, to typology. It is further extended to establish the properties of linguistic descriptions that account for variation in a unified way.