Child marriage usually refers to two separate social phenomena which are practiced in some societies. The first and more widespread practice is that of marrying a young child (generally defined as below the age of fifteen) to an adult. In practice, it is almost always a young girl married to a man.

The second practice is a form of arranged marriage in which the parents of two children from different families arrange a future marriage. In this practice, the individuals who become betrothed often do not meet one another until the wedding ceremony, which occurs when they are both considered to be of a marriageable age.

An increase in the advocation of human rights, whether as women's rights or as children's rights, has caused the traditions of child marriage to decrease greatly as it was considered unfair and dangerous for the children.

Child marriages may have many purposes. The aristocracy of some cultures tend to use child marriage among different factions or states as a method to secure political ties between them. For example, the son or daughter of the royal family of a weaker power would sometimes be arranged to marry into the royal family of a stronger neighbouring power, thus preventing itself from being assimilated. In the lower classes, if they were fortunate, families could use child marriages as means to gain financial ties with wealthier people, ensuring their successions.

In child betrothals, a child's parents arrange a match with the parents of a child from another family (social standing, wealth and expected education all play a part), thus unilaterally determining the child's future at a young age. It is thought by adherents that physical attraction is not a suitable foundation upon which to build a marriage and a family. A separate consideration is the age at which the wedding, as opposed to the engagement, takes place.

Families are able to cement political and/or financial ties by having their children inter-marry. The betrothal is considered a binding contract upon the families and the children. The breaking of a betrothal can have serious consequences both for the families and for the betrothed individuals themselves.