This chapter introduces key definitions and theories of student (or learner) engagement in both general education and second language (L2) learning contexts, and provides an overview of the book. It begins by reviewing fundamental frameworks of student engagement from educational psychology and general education, before narrowing the focus to multidimensional frameworks of L2 learner engagement in specific contexts (e.g., corrective feedback, collaborative writing). The discussion underscores the context-specific nature of learner engagement and establishes the rationale for conceptualising task engagement within task-based language teaching. The chapter concludes with an outline of the book’s purpose, structure, and content.