Preston's Preface identifies a gaping hole in the domain of SLA research: a “relative neglect ... of the insights to be gained from the quantitative study of interlanguage variation” (p. xiv). The book is intended to remedy that defect by providing (a) a clear rationale for the use of quantitative sociolinguistic methods to study interlanguage variation; (b) seven exemplary studies which use multivariate analysis of linguistic variation for this purpose; and (c) a practical, step-by-step manual on how to use VARBRUL computer programs to analyze variable interlanguage data. Readers of this book should come away with a very clear idea of why and when this sort of analysis should be done, and how to use this powerful tool in analyzing their own data.