The book Key Questions in Second Language Acquisition: An Introduction is an introduction to the field of Second Language Acquisition (SLA) which is organized around eight underlying questions driving the L2 research. This book reviews the empirical research in SLA on each question by illustrating the related assumptions and exemplary studies. Adopting the generative approach, the authors elaborate on how a second language is acquired and the outcome of acquisition. Theoretically, this book could offer novel insights and theoretical guidance for scholars and learners interested in Second Language Acquisition. Practically, the basic facts in the book may be informative for teachers' second language teaching in the classroom.