Most of the defamation law in the U.S. is state law, though there have been several Supreme Court cases that outline the constitutional limits on cases involving the two key types of defamation, libel and slander. Twenty-seven states currently have laws that provide criminal penalties for some types of libel. You can check out the First Amendment Center for specific state laws.
In practice, it is fairly rare for states to pursue criminal libel charges against individuals or media outlets. Some states have repealed the criminal sections of their libel laws, while others just never enforce them. The Supreme Court case Garrison v. Louisiana established that the government may only prosecute and individual for criminal libel if he makes false statements about a public figure while knowing that the statements are false or with reckless disregard to whether they are false.
In 2008, the Washington State Supreme Court invalidated the state’s criminal libel law, which hadn’t been enforced since 1925, in the case Parmelee v. O’Neel. The state criminal defamation statute permitted criminal prosecution for statements about public officials that were true but made without “good motives”, as well as for statements that were false but were made without actual malice. Neither of these types of prosecution is permitted under the Garrison standard, and the Washington court struck down the state law as vague and overly broad.
More information is available in the Depictions of Real People section.