This introductory chapter provides a rationale and overview for the book, which explores the diverse theoretical frameworks underpinning language learning. It addresses the central question of what drives language acquisition, examining theories ranging from behaviourism and innatism to constructivism, sociocultural perspectives, and emergentist approaches. Each theory offers unique insights into how language is acquired, processed, and used, emphasizing different aspects such as social interaction, cognitive processes, and cultural context. The chapter outlines the book’s structure, which progresses from foundational theories to more complex, integrative frameworks like complex dynamic systems theory. It also highlights the practical implications of these theories for language teaching, emphasizing the importance of addressing individual learner differences. By engaging with these perspectives, the book aims to foster a deeper understanding of language acquisition and inform effective, context-sensitive teaching practices. Readers are encouraged to reflect on their own experiences and critically apply these theories to their educational contexts.