Adolescents face many academic and emotional challenges in middle school, but notable differences are evident in how well they adapt. What predicts adolescents' academic and emotional outcomes during this period? One important factor might be adolescents' implicit theories about whether intelligence and emotions can change. The current study examines how these theories affect academic and emotional outcomes. One hundred fifteen students completed surveys throughout middle school, and their grades and course selections were obtained from school records. Students who believed that intelligence could be developed earned higher grades and were more likely to move to advanced math courses over time. Students who believed that emotions could be controlled reported fewer depressive symptoms and, if they began middle school with lower well-being, were more likely to feel better over time. These findings illustrate the power of adolescents' implicit theories, suggesting exciting new pathways for intervention.