International data stubbornly show that boys perform worse than girls in school and that women continue to be under-represented in university and professional careers related to STEM disciplines (Science, Technology, Engineering, and Mathematics). In this chapter, we discuss why these gaps should concern us and analyse the factors driving them. In the case of boys’ lower performance, we analyse the role of non-cognitive skills, families, and socioeconomic factors, as well as the impact of school and teachers, and labour market conditions. Regarding the under-representation of women in STEM, we examine the role of culture, social norms, and gender stereotypes, the absence of female role models among STEM teachers and professionals, and finally, the role of competitive contexts.