In Second Language Acquisition (SLA) there is clearly a need for detailed representations of the developing interlanguage competencies of learners. This volume is a 'festival' of techniques. The authors are familiar with up-to-date characterizations of the linguistic levels they study, with innovative methods of eliciting learner representations, and with the appropriate mathematical techniques required for handling such data (including, but not limited to, the VARBUL program of variationists. A bonus in this volume is a comprehensive VARBUL 'how to' for both DOS and Macintosh users.) 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 psycholinguistic bases of SLA.