Parametric variation in linguistic theory refers to the systematic grammatical variation permitted by the human language faculty. Although still widely assumed, the parametric theory of variation has in recent years been subject to re-evaluation and critique. The Null Subject Parameter, which determines among other things whether or not a language allows the suppression of subject pronouns, is one of the best-known and most widely discussed examples of a parameter. Nevertheless its status in current syntactic theory is highly controversial. This book is a defence of the parametric approach to linguistic variation, set within the framework of the Minimalist Program. It discusses syntactic variation in the light of recent developments in linguistic theory, focusing on issues such as the formal nature of minimalist parameters, the typology of null-subject language systems and the way in which parametric choices can be seen to underlie the synchronic and diachronic patterns observed in natural languages.