Language is acquired, comprehended, and produced in a rule-based fashion; nonetheless, variation in language is constantly observable—from alternating forms used by second and third language speakers to systematic changes in linguistic rules which eventually come to characterize entirely different language varieties. Therefore, understanding variation helps linguists understand the very forces that shape language itself. This book presents quantitative and qualitative research from interdisciplinary perspectives: second language acquisition, sociolinguistics, discourse studies and syntax. These ten chapters shed empirical light on the variables that result in systematic variation in language. From the influence of previously learned languages on the acquisition of a third language, to how social variables impact the phonetics of French political speaking styles, to how different types of comparatives in Jordanian Arabic can be distinguished by features within a syntactic hierarchy of functional projections, the chapters identify the linguistic factors which are behind the heterogeneity of structures in their individual topics of investigation.