Clear and present danger was a doctrine adopted by the Supreme Court of the United States to determine under what circumstances limits can be placed on First Amendment freedoms of speech, press, or assembly. Created by Justice Oliver Wendell Holmes Jr. to refine the bad tendency test, it was never fully adopted and both tests were ultimately replaced in 1969 with Brandenburg v. Ohio's "imminent lawless action" test.

Before the 20th century, most restrictions on free speech issues in the United States were imposed to prevent certain types of speech. Although certain kinds of speech continue to be prohibited in advance, dangerous speech started to be punished after the fact in the early 1900s, at a time when US courts primarily relied on a doctrine known as the bad tendency test. Rooted in English common law, the test permitted speech to be outlawed if it had a tendency to harm public welfare.

Antiwar protests during World War I gave rise to several important free speech cases related to sedition and inciting violence. In the 1919 case Schenck v. United States, the Supreme Court held that an antiwar activist did not have a First Amendment right to advocate draft resistance. In his majority opinion, Justice Oliver Wendell Holmes Jr. introduced the clear and present danger test, which would become an important concept in First Amendment law:

The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that the United States Congress has a right to prevent. It is a question of proximity and degree. When a nation is at war, many things that might be said in time of peace are such a hindrance to its effort that their utterance will not be endured so long as men fight, and that no Court could regard them as protected by any constitutional right.

In Frohwerk v. United States (1919) Justice Holmes summarized comments critical of U.S. wartime policies written by a newspaperman and stated about these comments the following: "It may be that all this might be said or written even in time of war in circumstances that would not make it a crime. We do not lose our right to condemn either measures or men because the country is at war." This statement "represents an important addendum to the original explication of the clear and present danger test in that it specifies that even during war, courts should regard criticism of government policies and officials as protected speech."

The Schenck decision did not formally adopt the clear and present danger test. Holmes later wrote that he intended the clear and present danger test to refine, not replace, the bad tendency test. Although sometimes mentioned in subsequent rulings, the clear and present danger test was never endorsed by the Supreme Court as a test to be used by lower courts when evaluating the constitutionality of legislation that regulated speech.

The Court continued to use the bad tendency test during the early 20th century in cases such as 1919's Abrams v. United States, which upheld the conviction of antiwar activists who passed out leaflets encouraging workers to impede the war effort. In Abrams, Holmes and Justice Brandeis dissented and encouraged the use of the clear and present test, which provided more protection for speech. In 1925's Gitlow v. New York, the Court made the First Amendment applicable against the states and upheld the conviction of Gitlow for publishing the "Left wing manifesto". Gitlow was decided based on the bad tendency test, but the majority decision acknowledged the validity of the clear and present danger test, yet concluded that its use was limited to Schenck-like situations where the speech was not specifically outlawed by the legislature.

Brandeis and Holmes again promoted the clear and present danger test, this time in a concurring opinion in 1927's Whitney v. California decision. The majority did not adopt or use the clear and present danger test, but the concurring opinion encouraged the Court to support greater protections for speech, and it suggested that "imminent danger" – a more restrictive wording than "present danger" – should be required before speech can be outlawed. After Whitney, the bad tendency test continued to be used by the Court in cases such as Stromberg v. California, which held that a 1919 California statute banning red flags was unconstitutional.