Boys lag behind girls in school across many western industrialised countries. On average, boys tend to be less engaged and perform worse than girls in secondary school. Yet efforts to close the gender gap may not be successful until we gain a precise understanding of the mechanisms contributing to the gender gap. This thesis presents three studies (N = 1,668) to unpack the motivational processes underlying the gender gap in school engagement and achievement, including gender differences in academic motivation (Study 1), gender differences in social motivation (Study 2), and a conflict between gender roles and school commitment (Study 3). Study 1 examines gendered patterns of academic beliefs and goals as well as the impact of these motivational patterns on student achievement. Latent profile analyses revealed four mindset-related motivational profiles: growth mindset-high mastery goals, growth mindset-high multiple goals, fixed mindset-high performance goals, and fixed mindset-low all goals. Compared to girls, boys were more often found in the two fixed mindset profiles. Membership in these profiles, in turn, predicted poorer achievement. Study 2 extends beyond a sole focus on academic motivation by investigating the joint role of academic and social motivation in explaining gender differences in school engagement. Compared to girls, boys endorsed more academic goals concerned with avoiding unfavourable judgement of ability and more social goals concerned with appearing cool in front of their peers. Furthermore, boys’ higher levels of academic self-handicapping were primarily explained by their greater concerns about peer status. Study 3 then moves beyond a binary perspective of gender to identify which boys and which girls are falling behind in school. Latent profile analyses identified seven subgroups of adolescent boys and girls, each displaying a unique pattern of gender role conformity. Young people who conformed to gendered ideals of appearance and behaviour showed the least adaptive patterns of motivation, engagement, and achievement. In contrast, those who rejected rigid constructions of gender had the most adaptive patterns of motivation, engagement, and achievement. Taken together, findings from these three studies provide concrete suggestions in terms of what factors to target as well as who to target in educational interventions to close the gender gap in school.