According to one authority, there are over 20 theories of SLA. This book attempts to help students without a substantial background in linguistics make sense of this diverse field by explaining and relating the most important of these. The second edition updates the original by dropping or shortening the discussion of older theories (Schumann's, 1986, acculturation model is now discussed in the chapter on historical antecedents) and updating the discussion of others (the expanded chapter on Vygotsky is particularly good). Chapter 1 contains a discussion of what an adequate theory of SLA would look like, pointing out, for example, the distinction between a property theory like Universal Grammar (UG) and a transition theory like connectionism; this distinction is referred to throughout the book. Chapter 2 offers a valuable account of the historical development of the SLA field, which includes discussions of behaviorism and the monitor model. Again, this preliminary groundwork is referred to in subsequent chapters.

Current SLA theories are grouped together under the following rubrics, with a chapter devoted to each: UG, cognitive approaches, functional/pragmatic perspectives, input and interaction approaches, sociocultural perspectives (Vygotskian theory), and sociolinguistic perspectives. The final chapter presents a summary and conclusions.

A major challenge in surveying a broad, interdisciplinary field is to present the details of complex theories clearly, concisely, and accurately. For the most part, the authors accomplish this goal. For example, the chapter on UG begins with a discussion of structure dependency illustrated by the familiar example of subject/verb inversion in yes-no questions. The authors then move on to more complex matters, including the inflection parameter, with illustrations from French and English. Empirical evidence for the operation of this parameter in SLA is then reviewed; the authors note that the evidence is inconclusive but that the UG framework enables researchers to develop clear hypotheses about this and other SLA issues. Throughout the chapter, the explanation of UG is clear and accurate, and it contains enough detail to be understood without much help from a teacher. The same cannot be said of all of the discussions, however. For example, the section on connectionism does not provide enough detail about how a connectionist network works. Students need to see an example of how a network learns through the mechanism of back propagation, which causes the connections to be reweighted. Therefore, this section—and a few others—might require considerable additional explanation in a classroom setting.

When reading literature reviews, the closer one is to a particular specialty, the more one notices errors and confusions. For example, in chapter 8, I noticed that in the reproduction of a table from Bailey (1996) displaying probabilistic VARBRUL weights for the production of word final t/d (as in missed and mist) by Chinese-speaking English learners, the authors do not explain that the weights in the different rows of the table are based on two different versions of the VARBRUL program and, therefore, cannot be directly compared. Without such an explanation, the table is misleading.

A strength of the book, on the other hand, is that the authors usually explain the connections, similarities, and differences between the theories that are grouped together in each chapter. For example, they point out how Long's (1983) interaction hypothesis extends Krashen's (1982) input hypothesis, and how Swain's (1995) output hypothesis expands on Long; they also contrast the entire discussion with similar concerns in first language acquisition. In chapters 4 and 8, however, the connections between the (very) different theories are not strongly drawn. Nevertheless, it is helpful that the authors present, for example, variation studies alongside ethnographic studies, so that students can see similar sociolinguistic themes addressed in succession, even if explicit connections between the theories are not made. Overall, this volume provides an accessible and authoritative introduction to SLA for beginning graduate students or advanced undergraduates.