Parameters have lain at the core of linguistic research in the generative tradition for decades. The theoretical questions they have raised are deep and broad: this reference text investigates how contemporary linguistics has best tried to answer them. This book looks at how parameters might be properly defined and what their locus might be :lexical information, functional heads, the computational system, the phonological branch of the grammar. What kind of data forms trigger acquisition of a parameter? Are parameters necessary or can we study languages without making reference to them? The questions looked at are not just theoretical: how can a theory of parameters be used to help understand second language acquisition, and what contributions can it make to the study of language typology? This is the right time to gather all this information, dispersed in many different kinds of publications by single authors and groups, into one comprehensive volume.