To understand the role of affect in classroom discussion, the authors asked 16 students taking a 3-hr seminar to participate in a semester-long grounded theory inquiry. Coding of classroom observations, stimulated recall interviews, and self-ratings from each class meeting resulted in a model of affect as a catalyst in students' experience of classroom discussion, both moment by moment and cumulatively across the semester. Influenced by personal and contextual factors, four main actions--attending, listening, talking, and tuning out--constituted the experience of discussion, with affect playing a key role. The consequences of this dynamic system of affect, cognition, and action in discussion were that students learned content, became more aware of social aspects, experienced different affective states, and changed their motivation to talk in future discussions.