Learner engagement is crucial for ensuring the quality of learning experiences, and yet the study of 'task engagement' has received relatively little empirical attention in the language education domain. This article reports on a study exploring the factors contributing to learners' engagement and disengagement during task performance in an English as a foreign language classroom. Thirty-seven learners performed 10 different speaking tasks implemented over a 10-week period in a Japanese university task-supported classroom. The participants' engagement was measured using a post-task questionnaire on perceived levels of anxiety, confidence, focus and desire to speak. Learners' written self-report reflections revealed complementary in-depth qualitative data about the underlying factors behind their engagement or otherwise during speaking tasks. The results revealed a variety of learner-level, lesson-level, task-level and post-task-level factors contributing to learners' engagement and disengagement in tasks. Findings indicate that certain task features such as the nature and purpose of the tasks, task repetition, familiar and easier task topics led to successful engagement. On the other hand, lack of social cohesion and motivational baggage were reported as primary factors in determining learners' silences and disengagement in tasks. The authors offer pedagogical implications for teachers on how speaking tasks can be better implemented to enhance engagement in foreign language classrooms.