Second language acquisition has an identity problem. It is a young field struggling to emerge from the parent fields of education and applied linguistics. In this book, the author proposes a way to help second language acquisition develop a systematic and coherent focus using the philosophy of science as the lens. The structure of the book allows a focus on areas of SLA of interest to many in the field. These include theory proliferation and comparative theory evaluation; the Critical Period Hypothesis and negative feedback; and the practice of "synthetic" language teaching. The controversial volume will be of interest to researchers, educators, and graduate students in second language acquisition, applied linguistics, TESOL, and linguistics programs. It may be recommended as additional reading for an introductory SLA course in order to stimulate class discussions. Following a preface, this book is divided into three parts. Part I, Theory, includes chapters: (1) Second Language Acquisition Theories; and (2) Problem-Solving and Theory Change in SLA. Part II, Research, presents: (3) Age Differences and the Sensitive Periods Controversy in SLA; and (4) Recasts in SLA: The Story So Far. Part III, Practice, concludes with (5) Texts, Tasks, and the Advanced Learner; and (6) SLA: Breaking the Siege.