[en] Depression is one of the most common, costly and severe psychopathologies worldwide. Its incidence, however, differs significantly between the sexes, and depression rates in women are twice those of men. Interestingly, this sex difference emerges during adolescence. Although the adolescent period is characterised by major physical and behavioural transformations, it is unclear why the incidence of depression increases so dramatically in girls during this otherwise generally healthy developmental period. Although psychological and environmental factors are also involved, we discuss the neuroendocrinological factors determining adolescent vulnerability to depression. In particular, we address the role of sex steroids in mood regulation, hypothalamic-pituitary-adrenal axis maturation and sexual differentiation of the brain, with a focus on hippocampal plasticity.