The purpose of this chapter is twofold. First, it introduces task-based language teaching (TBLT) as a prominent approach to second language instruction and reviews the key theories and empirical studies that have shaped its development. This section outlines the principles underpinning TBLT, reviews influential theoretical models, and considers how task-based research has advanced our understanding of L2 learning processes and outcomes. Second, the chapter reviews and compares the major frameworks of task engagement that have been widely cited in task-based research. Particular attention is given to how these frameworks conceptualise the multidimensional nature of task engagement and how they have been applied in empirical studies. By examining both TBLT theories and task engagement frameworks, the chapter provides a foundation for linking pedagogical practice with task engagement, setting the stage for the subsequent chapters in this book.