This thesis presents research investigating the association between gender nonconformity in childhood or adolescence and subsequent mental health outcomes, as well as potential factors which may help to explain these associations, using both quantitative and qualitative methods. First, a systematic review is presented which reviewed research investigating associations between childhood or adolescent gender nonconformity and common mental health difficulties, psychological wellbeing, self-harm, and suicidality. For studies using measures of self/other-perceived gender atypicality, 83% of studies in males (small-large effect sizes), 55% of studies in females (small-large), and 90% of studies in mixed samples (small-moderate) found greater gender atypicality was associated with poorer mental health. For studies instead using measures of gender-typed attributes without explicit reference to these being gender atypical (e.g. personality traits considered stereotypically masculine/feminine), 65% of studies in males (small-large) and 32% of studies in females (small-moderate) found less masculine (or more feminine) attributes were associated with poorer mental health. The existing literature has primarily used cross-sectional or retrospective designs, from which causation cannot be inferred; more longitudinal studies are needed.The second study used a prospective longitudinal design to assess the associations between stereotypically masculine and feminine behaviours measured in childhood and emotional or peer problems measured across childhood and adolescence in the ALSPAC birth cohort. For both males and females, more feminine-typed behaviour was associated with more emotional problems, and for males more feminine-typed behaviour was also associated with more peer problems. For females, more parent-rated masculine-typed behaviour, but more self-rated feminine-typed behaviour, were associated with more peer problems. Experiences of bullying, abuse, self-esteem, feeling different, and not feeling accepted may have contributed to some of these relationships.The third study used a prospective longitudinal design with the same birth cohort to assess the associations between gender-typed behaviour in childhood, experiences of abuse in childhood or adolescence, sexual orientation in adolescence, and symptoms of post-traumatic stress disorder (PTSD) at 23 years. This study found more masculine-typed parent-rated (but not self-rated) behaviour in females was associated with increased rates of self-reported sexual abuse, self-reported physical/psychological abuse, mother-reported abuse, and symptoms of PTSD, but that there were no associations for males. In females, experiences of each type of abuse report independently mediated the association between parent-rated gender-typed behaviour at 30 months and PTSD at 23 years. Self-reported physical/psychological abuse mediated this association even when adjusting for sexual orientation; however, mother-reported abuse was no longer a significant mediator once self-reported physical/psychological abuse was included in the model.The fourth study used a qualitative design to gain a more in-depth understanding of experiences associated with gender nonconformity in childhood or adolescence. Young adults were interviewed about their experiences of gender nonconformity while they were growing up, and thematic analysis of their reports was conducted. Themes related to difficult experiences included: enforced gender segregation, feeling different, puberty, bullying, pressure to conform, unwanted sexual experiences, developing awareness of sexuality, and identifying as a different gender. These experiences went on to have impacts on changing behaviour to conform with others, distress about the current situation and future, mental health problems and self-harm, not feeling accepted, feeling bad about one’s self and body, keeping to oneself due to (fear of) negative reactions, difficult relationships with family, and wishing to change sex. Participants also highlighted factors that were or would have been helpful, including acceptance of gender nonconformity, meeting similar others, and LGBTQ+-specific support, as well as more general coping mechanisms.Together, these studies have identified several potential avenues by which to help prevent gender nonconforming young people from developing mental health difficulties. The results also indicated that experiences may be different depending on sex assigned at birth. The implications of these findings are discussed.