PART I. Studies on Disparities CHAPTER 2. Sexual Identity Disparities in Mental Health Among UK Adults, US Adults, and US Adolescents: Examining Heterogeneity by Race/Ethnicity 2.1. Introduction 2.2. Method 2.3. Results 2.4. Discussion 2.5. Conclusion CHAPTER 3. Associations of Dating Experiences, Dating Violence, Sexual Harassment, and Assault with Alcohol Use Among Sexual and Gender Minority Adolescents3.1. Introduction 3.2. Methods 3.3. Results 3.4. Discussion 3.5. Conclusion PART II. Studies on Mechanisms CHAPTER 4. Sexual and Gender Identity-Based Microaggressions: Differences by Sexual and Gender Identity, and Sex Assigned at Birth Among Dutch Youth 4.1. Introduction 4.2. Study 1 4.3. Study 2 4.4. Discussion 4.5. Conclusion CHAPTER 5. Explaining Health Disparities Between Heterosexual and LGB Adolescents by Integrating the Minority Stress and Psychological Mediation Frameworks: Findings from the TRAILS Study 5.1. Introduction 5.2. Methods 5.3. Results 5.4. Discussion 5.5. Conclusion CHAPTER 6.