This volume corrects the relative neglect in Second Language Acquisition studies of the quantitative study of language variation and provides insights into such issues as language transfer, acquisition through exposure, language universals, learner's age and so forth. These studies bolster the idea that a full account of SLA development (and, hence, a "theory of SLA") must be built on not only detailed accounts of interlanguage data but also on a wide appeal to factors which govern the psycholinguistic bases of SLA. An important addition to the volume is a comprehensive guide to both the DOS and Macintosh versions of the VARBRUL statistical program used by variationists.