Parents are considered the natural custodians of their children. As such, courts typically do not deny parents custody except occasionally during divorce proceedings or when a parent is found to be unfit. A guardian is normally appointed for a child when both parents are unavailable or unfit, or for an adult if he is mentally or physically disabled to the extent he cannot provide for his own basic needs. Custodians are never appointed for adults. The legal standard for appointing either a custodian or a guardian is the "best interests" of the ward.
A person with legal custody over a child, even if someone other than a parent (a stepmother, for example), exercises broad authority over that child equivalent to the responsibilities traditionally held by a parent . A guardian's authority, by contrast, may be limited as circumstances dictate – for example, a guardian's authority may be limited to making medical treatment decisions on behalf of a child. Even if a guardian is appointed for a child, a parent may retain custody rights, such as visitation, which cannot be overruled by the guardian. The authority of a guardian over a disabled adult is limited to the least restrictive authority necessary to provide for the adult's best interests.
Custody over a minor terminates automatically when the child reaches the age of majority – 18 in most states. Guardianship of a minor, by contrast, lasts only while both of the child's parents are unfit or unavailable. A parent may petition a court to terminate guardianship of a minor in favor of parental custody; a court will grant the petition unless the parent is shown to be unfit to care for the child. Although custodianship may be permanent unless or until circumstances intervene, a guardianship may be temporary in nature. For example, a guardian may be appointed to make medical treatment decisions on behalf of a child while his parents are on temporary assignment overseas.
A biological parent requires no legal procedure to obtain custody of his child. Adoption procedures automatically confer custody to the adoptive parent. Except for these circumstances, custody requires a court order while guardianship always requires one. In both types of arrangements, parties opposed to a custody or guardianship petition may oppose it at an adversarial hearing.