Defenses.

Even if a statement is defamatory, there are circumstances in which such statements are permissible in law.

Truth.

Proving adverse public character statements to be true is often the best defense against a prosecution for libel or defamation. Statements of opinion that cannot be proven true or false will likely need to apply some other kind of defense.

Another important aspect of defamation is the difference between fact and opinion. Statements made as "facts" are frequently actionable defamation. Statements of opinion or pure opinion are not actionable. Some jurisdictions decline to recognize any legal distinction between fact and opinion. To win damages in a libel case, the plaintiff must show that the statements were "statements of fact or mixed statements of opinion and fact." Conversely, a typical defense to defamation is that the statements are opinion, relying on opinion privilege. One of the major tests to distinguish whether a statement is fact or opinion is whether the statement can be proved true or false in a court of law. If the statement can be proved true or false, then, on that basis, the case will be heard by a jury to determine whether it is true or false. If the statement cannot be proved true or false, the court may dismiss the libel case without it ever going to a jury to find facts in the case.

Under English common law, proving the truth of the allegation was originally a valid defense only in civil libel cases. Criminal libel was construed as an offense against the public at large based on the tendency of the libel to provoke breach of peace, rather than being a crime based upon the actual defamation per se; its veracity was therefore considered irrelevant. Section 6 of the Libel Act 1843 allowed the proven truth of the allegation to be used as a valid defense in criminal libel cases, but only if the defendant also demonstrated that publication was for the "Public Benefit".

In some systems, however, notably the Philippines, truth alone is not a defense.

It is also necessary in these cases to show that there is a well-founded public interest in the specific information being widely known, and this may be the case even for public figures. Public interest is generally not "what the public is interested in", but rather "what is in the interest of the public".

Noonan v Staples is sometimes cited as precedent that truth is not always a defense to libel in the U.S., but the case is actually not valid precedent on that issue because Staples did not argue First Amendment protection, which is one theory for truth as complete defense, for its statements. The court assumed in this case that the Massachusetts law was constitutional under the First Amendment without it being argued by the parties.

In a 2012 ruling involving Philippine libel law, the United Nations Commission on Human Rights commented, "Penal defamation laws should include defense of truth."