McCarthyism, also known as the second Red Scare, was a period of political repression and persecution of left-wing individuals in the United States. It was a campaign that spread fear of alleged communist and Soviet influence on American institutions and of Soviet espionage. The term "McCarthyism" was first published in late March 1950 in The Christian Science Monitor, after Senator Joseph McCarthy claimed to have a list of members of the Communist Party USA working in the State Department. The most notable examples of McCarthyism include the investigations of alleged communists that were conducted by Senator McCarthy, and the hearings conducted by the House Un-American Activities Committee (HUAC).

The historical period that came to be known as the McCarthy era began well before Joseph McCarthys own involvement in it. Many factors contributed to McCarthyism, some of them with roots in the First Red Scare (1917–20), inspired by communisms emergence as a recognized political force and widespread social disruption in the United States related to unionizing and anarchist activities.

McCarthyism both reached its peak and began its decline during the "McCarthy hearings": 36 days of televised investigative hearings led by McCarthy in 1954. The hearings reached their climax when McCarthy suggested that the Armys lawyer, Joseph Welch, had employed a man who at one time had belonged to a communist front group. Welchs rebuke to the senator -- "Have you no sense of decency, sir, at long last? Have you left no sense of decency?" -- discredited McCarthy and helped to turn the tide of public opinion against him. McCarthy was censured for his conduct by the Senate, and in 1957 he died.