This study adopts a flow theory perspective to investigate the learning engagement levels of 224 Korean high school students during EFL classes and the factors that may relate to differences in their engagement. Engagement was defined as the simultaneous experience of concentration, interest, and enjoyment. The experience sampling method was employed to measure 10th and 11th graders’ appraisals of instructional activities. Each participant was sampled twice randomly during each of three observed lessons. Momentary levels of engagement were examined in relation to students’ appraisals of activity qualities (importance for future goals, difficulty, possessed skills/competence in the area of the activity), and contextual factors (gender, grade, teacher, ability-track). Descriptive analyses, ANOVAs, and t-tests revealed that only 13% of the students were optimally engaged in lessons that were predominantly teacher-centered; increased engagement was associated with students’ appraisals of activity importance for future goals and with feelings of competence; and learner group engagement differed according to who taught them. Some teachers seemed to use an approach more conducive to engagement.