Why do some adolescents respond to peer conflict, exclusion or victimization vengefully, while others seek more positive solutions? Five studies in the dissertation investigated the role of implicit theories of personality in predicting 9th and 10th grade adolescents' violent or vengeful responses to peer conflicts, and extended this theoretical framework to understand stress and academic engagement at the transition to high school. A greater belief that traits are fixed (an entity theory) predicted a stronger desire for revenge after a variety of recalled peer conflicts (Study 1) and after a hypothetical conflict that specifically involved bullying (Study 2). Study 3 experimentally induced a belief in the potential for change (an incremental theory), which resulted in a reduced desire to seek revenge. This effect was mediated by changes in "bad person" attributions about the perpetrators, feelings of shame and hatred, and the belief that vengeful ideation is an effective emotion-regulation strategy. Next, Study 4 examined whether an implicit theories intervention could change aggression. Adolescents are often resistant to interventions that reduce aggression in children. At the same time, they are developing stronger beliefs in the fixed nature of personal characteristics. Study 4's intervention addressed these beliefs. A randomized field experiment with a diverse sample of high school students (N = 230) tested the impact of a 6-session intervention teaching an incremental theory (a belief in the potential for personal change). Compared to No-Treatment and Coping Skills control groups, the Incremental Theory group behaved significantly less aggressively and more prosocially one month post-intervention and exhibited fewer conduct problems three months post-intervention. Interestingly, the Incremental Theory and the Coping Skills interventions also eliminated the association between peer victimization and depressive symptoms. Study 5 extended the theoretical model to include the effects of implicit theories on stress and academic achievement. The transition to high school is often accompanied by stress and declining grades. One explanation is that many students think that the social adversities they face—such as rejection, exclusion or victimization—are permanent. To test this hypothesis, Study 5 included a double-blind, randomized field experiment that taught students the idea that people have the capacity for change. This incremental theory intervention, which lasted two class periods at the beginning of the school year, reduced stress and improved Math and English grades over eight months, until the end of the school year. These findings highlight the importance of social processes in shaping academic performance during times of stress. Together, the findings from Studies 1-5 illuminate the social-cognitive processes underlying reactions to conflict and suggest potential avenues for reducing violent retaliation in adolescents. The dissertation concludes with an essay discussing how seemingly "small" psychological interventions with adolescents—that is, brief exercises that target students' thoughts, feelings, and beliefs in and about school—can lead to large gains in student achievement and reduce aggression even months later. It emphasizes that such interventions have lasting effects because they target students' subjective experiences in school, because they use persuasive yet stealthy methods for conveying psychological ideas, and because they tap into recursive processes present in educational environments. By understanding psychological interventions as powerful but context-dependent tools, researchers will be better equipped to take them to scale.