This thesis focuses upon the first unmarried and childless partnership dissolution as a new event in the transition to adulthood. Since the 1980s, the first cohabiting couple is part of an extensive youth and a weakened synchronization of events that define the entry into adulthood. In Belgium and France, young adults’ autonomy comes up against economic uncertainties. In this context, partnership dissolution occurs in various pathways into adulthood. This thesis addresses the social mechanisms and the consequences of union dissolution during youth. The work is based on several surveys (GGS, EU-SILC, Épic) and administrative data (Belgian register, EDP). Union dissolution is a new stage in the affirmation of individual autonomy, contributing to the postponement of marriage and first birth. Material and economic conditions particularly determine the outcome of a first unmarried and childless union. For students and university graduates, partnership dissolution fosters emancipation when the relationship conflicts with expectations or professional plans in early careers, especially women’s ones. For low‑skilled young adults, an unpredictable labor market and economic vulnerabilities contribute to partnership dissolution, which creates tensions around residential independence, especially among men. Although the place of separation in young people's lives is socially diverse, partnership and parental norms are barely reshaped. Regardless of gender or education, moving in with a second partner is a common event after the separation of an unmarried and childless union. The second union offers a privileged framework for a first birth or, less often, a first marriage. ; Cette thèse a pour objet la première séparation d’une union hors mariage et sans enfant, analysée comme un nouvel événement des parcours d’entrée dans l’âge adulte des femmes et des hommes. Depuis les années 1980, la fragilité de la première vie en couple cohabitant s’inscrit dans un allongement de la jeunesse et une ...