The papers in this collection share a common interest in the empirical, theoretical and meta-theoretical aspects of the ‘internal-external’ (‘formal-functional’) debate in linguistic theory. The primary aim of this volume is to initiate cooperation between internationally renowned generative and variationist linguists with a view to developing an innovative and more cohesive approach to syntactic variation. The present volume contains treatments incorporating the analysis of external factors into accounts focusing on the internal linguistic conditioning of syntactic variation and change cross-linguistically. As such, it offers novel approaches to three key areas of current linguistic debate, viz. (1) Methodological practices, (2) Theoretical applications and (3) Modularity. The volume is, therefore, an important achievement for the progress of linguistic theory more generally and it is an even more crucial milestone in the coming-of-age of ‘Socio-Syntax’ as a discipline in its own right.