Internationally.

Article 17 of the United Nations International Covenant on Civil and Political Rights states

1. No one shall be subjected to arbitrary or unlawful interference with his privacy, family, home or correspondence, nor to unlawful attacks on his honor and reputation.

2. Everyone has the right to the protection of the law against such interference or attacks.

United States.

The origins of U.S. defamation law pre-date the American Revolution; one famous 1734 case involving John Peter Zenger sowed the seed for the later establishment of truth as an absolute defense against libel charges. The outcome of the case is one of jury nullification, and not a case where the defense acquitted itself as a matter of law, as before the Zenger case defamation law had not provided the defense of truth.

Though the First Amendment of the U.S. Constitution was designed to protect freedom of the press, for most of the history of the United States, the Supreme Court neglected to apply the First Amendment to libel cases involving media defendants. This left libel laws, based upon the traditional common law of defamation inherited from the English legal system, mixed across the states. The 1964 case New York Times Co. v Sullivan dramatically altered the nature of libel law in the United States by elevating the fault element for public officials to actual malice – that is, public figures could win a libel suit only if they could demonstrate the publisher's "knowledge that the information was false" or that the information was published "with reckless disregard of whether it was false or not".

Later the Supreme Court held that statements that are so ridiculous to be clearly not true are protected from libel claims, as are statements of opinion relating to matters of public concern that do not contain a provably false factual connotation. Subsequent state and federal cases have addressed defamation law and the Internet.

Defamation law in the United States is much less plaintiff-friendly than its counterparts in European and the Commonwealth countries. A comprehensive discussion of what is and is not libel or slander under United States law is difficult, as the definition differs between different states and is further affected by federal law. Some states codify what constitutes slander and libel together, merging the concepts into a single defamation law.

Civil defamation.

Although laws vary by state, in the United States a defamation action typically requires that a plaintiff claiming defamation prove that the defendant:

1. made a false and defamatory statement concerning the plaintiff;

2. shared the statement with a third party (that is, somebody other than the person defamed by the statement);

3. if the defamatory matter is of public concern, acted in a manner which amounted at least to negligence on the part of the defendant; and

4. caused damages to the plaintiff.

American writers and publishers are protected from foreign libel judgments not compliant with the US First Amendment, or libel tourism, by the SPEECH Act, which was passed by the 111th United States Congress and signed into law by President Barack Obama in 2010. It is based on the New York State 2008 Libel Terrorism Protection Act (also known as "Rachel's Law", after Rachel Ehrenfeld who initiated the state and federal laws). Both the New York state law and the federal law were passed unanimously.

Defenses to defamation that may defeat a lawsuit, including possible dismissal before trial, include the statement being one of opinion rather than fact or being "fair comment and criticism". Truth is always a defense.