Divorce may be defined as the legal ending of a marriage. Married couples do not have a legal or a constitutional right to seek a divorce, but states commonly grant divorces because to force people to stay married would be against public policy.

No-fault and fault are the two types of grounds, or justifications, for divorce in the United States. In some states, no-fault is the only acceptable justification for a divorce, even if fault grounds exist within the marriage. The other states allow the parties to select either a no-fault or a fault divorce. The Cornell University Law School website gives a comprehensive overview of divorce and separation law in the United States.

The United States first adopted the concept of a no-fault divorce in the late 1960's, and nearly all of the states currently allow for some form of it. A no-fault divorce is one in which the parties do not legally fault, or blame, one another for the divorce. Some examples of the bases for a no-fault divorce include the following: “irreconcilable differences,” “irretrievable breakdown,” or “incompatibility.” One spouse cannot usually stop a no-fault divorce, simply because when one spouse wants a divorce and the other spouse does not, this constitutes irreconcilable differences, which justifies a no-fault divorce.

Many states require a "cooling-off period" of separation for a specific length of time before no-fault divorce proceedings can begin. During this time, the members of the couple are required to live separate and apart with the intent that the separation will become permanent. The duration of the cooling-off period varies according to state law.  

Some states also allow fault grounds for divorce. Common fault grounds may include: cruelty, desertion, adultery, prison confinement, habitual intemperance (drunkenness or drug addiction), and impotency. If both parties are proven to be at fault, courts will typically grant the divorce to the party least at fault by using the doctrine of “comparative rectitude.” Some states may use the comparative fault of the parties as a basis for granting a greater alimony award or a larger segment of marital property to the spouse proven to be least at fault. Couples might seek a divorce based on fault because they do not want to delay the divorce by complying with their state’s mandatory separation requirements for a no-fault divorce.

There are several “defenses,” or denials, that are sometimes used with regard to fault divorces, including the following: collusion, connivance, condonation, and provocation. These defenses are not used often, perhaps because they require witnesses and additional court time, which is expensive and can drag out the divorce. Another reason may be that even if the defense is proven, courts will typically still grant the divorce rather than require couples to remain married when at least one spouse does not wish to do so.

Collusion may be committed by a couple who does not want to wait the mandatory separation period in their state, and instead, attempts to falsely manufacture fault grounds for the divorce. When one spouse sets the other spouse up for wrongdoing and then later uses it as a basis for a fault divorce, this is called connivance. The condonation defense is sometimes used when a party condoned a certain spousal behavior during the marriage and then attempts to use it as fault ground for divorce. Provocation may be used as a defense if one spouse provokes wrongdoing on the part of the other spouse.

Divorce laws in general, as well as the specific grounds required for divorce, vary widely across the United States. The American Bar Association provides a large variety of information regarding divorce requirements state by state on its Section of Family Law website. A chart comparing all 50 states can be found at abanet.org.