This book is for readers who wish to understand Second Language Acquisition (SLA) in a clear, practical, and academically grounded way. It is intended especially for master’s students in English language studies, applied linguistics, TESOL, and language education; teachers of English who want to connect classroom practice with research; beginning researchers who are entering the field of SLA; and teacher educators, curriculum designers, and language professionals who need a systematic foundation in how additional languages are learned. Although the book pays particular attention to the Vietnamese EFL context, its discussions also engage with the wider international literature in SLA.