The House Un-American Activities Committee (HUAC) was an investigative committee in the United States House of Representatives which was meant to look into suspected cases of subversion and disloyalty to the United States government. The actions of HUAC attracted a great deal of public attention while the committee was active, and it continues to be a topic of discussion and research among academics. The long arm of the committee is often referenced in popular slang, as well.

In 1934, the House established the Special Committee on Un-American Activities, supposedly to investigate Americans of German origin and to keep an eye on subversive behavior in the United States. In 1938, the committee acquired permanent status, changing its name to HUAC and appointing Martin Dies as its chair. The HUAC was active well through the 1960s, although its influence had begun to decline by 1969, when it was renamed the House Committee on Internal Security; it was dissolved entirely in 1975.

Although the HUAC was supposedly supposed to investigate all potential subversion, conspiracies, disloyalty, and other unacceptable activity, it focused primarily on people with politics to the left of the political spectrum, and was often accused of an anti-Socialist and anti-Communist bias. Dies himself was an active member of the Klu Klux Klan, and the HUAC did not often prosecute or question people with conservative politics, including right-wing extremists and supporters of the Nazi Party. One of the most famous HUAC cases was that of Alger Hiss, who was sentenced to jail for perjury, despite being a notable politician, diplomat, and lecturer.

Members of HUAC had a great deal of power, with the ability to issue subpoenas to compel testimony. Some notable individuals refused to testify for HUAC, for fear of implicating themselves and others, and they were blacklisted; the Hollywood Blacklist of the HUAC included a number of prominent celebrities. Once blacklisted, people had trouble obtaining employment, social services, and other support.

Many people associate the famous anti-Communist Senator Joseph McCarthy with the HUAC. Although McCarthy's actions and those of the HUAC were often very similar, McCarthy was not, in fact, affiliated with this organization, as he was a member of the Senate, not the House. He was certainly inspired by some of the techniques used by members of the committee, however, and the combined force of the two undoubtedly shaped the American political landscape.