Thesis: Family has known great transformations in the last decades in a large number of OECD countries. On one hand, fertility rates have decreased. On the other hand, the number of separations has increased sharply. This thesis asks whether these major changes of family structure affect child’s human capital, being a new source of inequalities. Using very large datasets and micro-econometric methods, the three chapters present original empirical evidence on whether parental separation and family size impact individual’s human capital. The first chapter studies the effect of parental separation in France on individual’s achievement, and find a negative effect of parental separation on individual’s educational attainment and social position. Using an American dataset, the second chapter asks whether this effect is driven by changes in time spent with parents, and find that 30% of the effect of parental separation on socio-emotional skills is explained by the decrease in time spent with at least one parent present. The third chapter accounts for another aspect of family structure: the number of children. Using a British dataset, we find that having a second sibling in the United Kingdom decreases the child’s socio-emotional skills, especially for girls.