This study investigated students' activating positive affective experiences in mathematics lessons, their antecedents, their cognitive and motivational consequences, as well as their effect on achievement. The participants were 682 Grade 8 and 9 students from 37 classes from Germany and Switzerland who participated in a video study of lessons on the Pythagorean theorem. Control and value beliefs as well as autonomy support proved to be relevant predictors of positive affective experiences. Moreover, positive affective experiences were associated with more cognitive activity during the lessons, which for its part predicted achievement in the posttest. In a follow-up test, there was also an indirect effect of positive affective experiences on achievement, mediated by cognitive activity and expectancy of success. (Contains 7 tables.)