This book is written in order to help undergraduate students and trainee teachers to reflect on certain topics and key issues related to second language acquisition. Despite the proliferation of books and introductory courses in second language acquisition, most of these books very often provide a very complex account of theoretical views in second language acquisition and sometimes fail to emphasise the crucial interplay between how people learn languages and what is the most effective way to teach languages. The overall purpose of this book is to provide an overview of second language acquisition research and theories by identifying the main key issues in this field and by highlighting the relevance of this research for classroom implications. The study of second language acquisition is a rich and varied enterprise, carried out by researchers, whose interests and training often lie in broader disciplines of linguistics, psychology, sociology, and education. Readers will be encouraged to critically reflect on the presented content through self-engaging thinking activities in the form of questions, matching activities, choices and conclusions about the implications of SLA theories to the real world applications.