Although teachers' knowledge of student emotion is not typically integrated into studies of teachers' pedagogical content knowledge, this study uses the philosophy of emotion, recent teacher knowledge research, and a case study of one middle-school physical education teacher to illustrate the point that how teachers understand student emotion is inextricably linked to their thinking and decisions about educational content, curriculum, and pedagogy. Data were collected during 4 months of observations and interviews and were analyzed using constant comparison. Three themes are used to show how this teacher's interpretations of student emotion influenced her selecting, ordering, and formulating of curriculum units, her pedagogical maneuvering during lessons to facilitate learning, and her interactions with individual students and groups of students. The discussion centers on the need to expand current conceptions of teachers' pedagogical content knowledge, and the importance of emotional understanding in teaching. Future directions for research into emotion and teaching are suggested.