In general, parents owe their children a legal duty of financial support until the child reaches the age of majority (usually either 18 or 21 years old, depending on state law) or becomes self-supporting/emancipated. When only one parent has primary custody of the child, the other parent's obligation for financial support is usually fulfilled through the payment of child support. Child support is owed whether the child lives with his or her other parent or a third party. It doesn't matter from a legal perspective whether or not the person with whom the child lives could technically afford to support the child on his or her own; the legal duty of financial support still exists for both parents. Depending on the state, child support may be owed even if the parents share custody.

Each state has adopted its own set of guidelines for determining child support. While individual guidelines differ, most arrive at the amount of support owed through a consideration of the needs of the child, the amount of time the child spends with each parent and the income of the paying parent. Family courts use the guidelines to establish the amount of support required. The guideline amounts are presumed to be proper unless persuasive evidence to the contrary exists.

The duty to pay child support generally starts with an order for support from a state family court. The order may be issued as part of a temporary or final divorce proceeding. If the parents were unmarried, then paternity must be established before an order for support will be issued.

Child support payments are usually due at specific times each month and in many jurisdictions may be directly withheld from the paying parent's wages. In most states, the paying parent may be able to make his or her payments to a child support registry that will forward the payments to the custodial parent and keep track of payments that are made.

If child support payments aren't made as scheduled, a variety of measures exist to collect past due amounts (called "arrears" or "arrearages"), and to protect against future non-payments. To encourage prompt child support payment, many states have laws that allow a family court judge to suspend professional or business licenses, to take away drivers' and recreational licenses, to require prepayment of future child support, or to order incarceration for the failure to make court-ordered child support payments.

Each state also has an office of child support enforcement. These federally supported state agencies help locate responsible parents, work with family courts to create child support orders and ensure payment. There are also federal laws that criminalize non-payment of child support when the parents live in different states.

Both the parent receiving child support and the parent paying child support may request changes to a child support order. Some states require regular review of existing child support orders while others review child support orders only upon request. Parents receiving support may have the amount increased upon a showing that the paying parent's income has increased, especially if the current amount of ordered support does not meet the child's needs. Support may also be increased because of a child's specific needs for things like medical treatment, therapy, tutoring, or orthodontic care.

Paying parents may be able to decrease the amount of future support payments if they face the loss of a job, a significant reduction in income or when the custodial parent's income increases. Federal law prohibits states from forgiving past due child support payments, however. Courts are reluctant to reduce child support awards without good cause, and paying parents may have an earning capacity imputed to them whether or not their actual earnings reflect that amount. This is often the case if the court feels that a parent from whom support has been ordered is voluntarily underemployed or unemployed just to avoid paying child support.