Child marriage is the formal union between two children or adolescents who have not reached the legal age of adulthood and are considered minors. It was a common practice among many different ethnic groups to ensure the financial security of young girls, the well-being of the young couple, and the continuity of cultural practices of the group. A critical examination of long-term effects of child marriage through developmental and sociological lenses highlights both the positive, and most likely negative effects of child marriage especially on young girls. This entry reviews rationale for child marriage, how child marriage is defined internationally, and the effects of early marriage on young girls. First, this entry discusses how the legal age of majority is connected to marriage.