The authors explored the relationship between motivation and affect in upper elementary mathematics classes from the perspective of flow theory. They also investigated the relationship between students' motivation and teachers' instructional practices. Students' reported classroom experiences formed 4 factors--Social Affect, Personal Affect, Efficacy, and Challenge/Importance. On the basis of student reports, the authors concluded that (a) affect is essential to student experience in mathematics lessons, (b) skill is perceived in conjunction with affective variables, (c) challenge is identified as a threat to students' efficacy, and (d) importance of a task is more relevant to motivation than is its challenge. A qualitative investigation of teacher instructional discourse suggested that the following teacher practices related to student motivation: (a) provision of substantive feedback and clarification of concepts; (b) support for autonomy, cooperation, and social relatedness; and (c) emphasis on learning for its own sake. Results suggest that emphasizing the balance of challenge and skill, supporting self-efficacy and value for mathematics, and fostering positive affect can enhance student motivation in the classroom. (Contains 4 tables, 1 figure, and 2 notes.)