This chapter reviews 53 research articles on task engagement indexed in Scopus, examining their topics, scope, and major findings. The review reveals an emerging trend of task engagement research within TBLT from 2000 to 2025, particularly in L2 speaking contexts. These studies address three main areas: task-related factors (e.g., learner choice, task complexity, task type, and task modality), learner-related factors (e.g., individual differences and interpersonal dynamics), and teachers’ conceptualisation, assessment, and practice of task engagement. Findings are summarised within these domains, and research gaps are identified. Specifically, although task factors have been explored, the results are not always consistent, highlighting the need for further investigation to explain these mixed findings. In addition, a broader range of learner factors requires examination to understand how learners with different characteristics engage in tasks. Finally, teachers’ understanding and practice of task engagement remain underexplored. The review’s findings help provides a rationale for the empirical studies presented in Parts II and III.