A prior restraint is an official government restriction of speech prior to publication. Prior restraints are viewed by the U.S. Supreme Court as “the most serious and the least tolerable infringement on First Amendment rights.” The Court repeatedly has found that such attempts to censor the media are presumed unconstitutional.
Prior restraints often arise when a judge is concerned about the effect of publicity on a defendant's fair trial rights. In such contexts, judges must apply the “clear and present danger” test, examining whether “the gravity of the ‘evil,’ discounted by its improbability, justifies such invasion of free speech.” The court must look at whether other less restrictive measures would have alleviated the effects of pretrial publicity, and whether a restraining order would be effective in preventing the threatened danger.
The "clear and present danger" test also applies in the national security and law enforcement areas. Courts have been reluctant to issue restraints over privacy concerns, and won't do so when information is in the public sphere. Restraints over corporate information are also rare. Some statutes restraint the media from releasing some information.
Restraints on Internet speech follow the same rules, although particular speech can often be restrained if it has already been adjudged as libelous.