What are the main limitations on First Amendment freedom of speech in the United States?

The First Amendment prohibits the government (federal, state, and local) from restricting speech based on content or viewpoint, with well-established exceptions. Unprotected categories include true threats (serious statements of intent to commit violence), incitement to imminent lawless action (the Brandenburg v. Ohio standard), defamation (false statements of fact that damage a reputation), obscenity meeting the Miller v. California three-part test, and fraud. Critically, the First Amendment only protects against government action, not private action. A private employer, a social media company, or a private university is generally free to restrict speech on their platforms or premises without a First Amendment violation.