The "clear and present danger" rule is a test used to determine the permissible limits of freedom of expression. It asks whether the words used in particular circumstances create a clear and present danger of bringing about substantive evils that the State has the right to prevent. It is considered the most libertarian of the tests.

Under this rule, "clear" implies a causal connection between the expression and the apprehended danger, while "present" refers to the time element, meaning the danger must be imminent and immediate. The danger should not merely be probable, but very likely inevitable. The substantive evil must be extremely serious and the degree of imminence extremely high before expression can be punished. Fear alone is not enough to justify suppression; there must be reasonable grounds to believe serious evil will result and that the danger is imminent and serious.

This test is applied to content-based restrictions on speech and media, including broadcast media, where the government bears a heavy burden to justify the restraint. It has been applied in cases concerning seditious speech and denial of permits for public assemblies.

Justice Holmes used an analogy about shouting "fire" when explaining the clear and present danger rule.

He stated that the most stringent protection of free speech would not protect a man in falsely shouting "Fire, fire" in a crowded theater and causing a panic. This illustrates that even strongly protected speech has limits when it creates an immediate and serious danger.