A nuclear family is traditionally defined as consisting of a mother and father in a first-time marriage and their children. This family structure also has been referred to as the traditional American family, the elementary family, and the basic unit of society, and has often been deemed the norm for family structure. Historical trends have shown a deviation from the nuclear family, leading to an expanded definition of the family in society. For purposes of family therapy, a nuclear family is comprised of caretakers and children in one household. This entry outlines the traditional structure of the nuclear family, discusses trends leading to changes in family structure and roles, and details the expansion of the definition of the nuclear family.