Adultery is extramarital sex that willfully and maliciously interferes with a marriage. Though all states now offer a no-fault ground for divorce, meaning proving fault is not required to obtain a divorce, in some jurisdictions, proof of adultery is still sufficient legal grounds for divorce.
Historically, adultery and divorce were much more closely correlated than they are today in the eyes of the law. At one time, the innocent party had to prove that his or her spouse committed some significant wrongdoing in order to get a divorce or a separation. This meant that the innocent party (who became the plaintiff) had to show that adultery, abandonment, cruelty, or some other wrongdoing occurred to justify the divorce.
Before no-fault divorce, adultery and divorce was treated much differently and also depended on who was committing the adultery. While most states no longer consider adultery a crime, they do recognize adultery as a viable ground for divorce. States with no-fault divorce laws do not require that the innocent party show proof that the other party committed adultery in order to grant divorce. In no-fault divorce cases, there are certain legal requirements that must be met but these do not include proof of fault.