"On the Cult of Personality and Its Consequences" (Russian: «О культе личности и его последствиях») was a report by Soviet leader Nikita Khrushchev made to the 20th Party Congress of the Communist Party of the Soviet Union on 25 February 1956. Khrushchev's speech was sharply critical of the reign of deceased General Secretary and Premier Joseph Stalin, particularly with respect to the purges of the Communist Party of the Soviet Union which had especially marked the last years of the 1930s. Khrushchev charged Stalin with having fostered a leadership personality cult despite ostensibly maintaining support for the ideals of communism. The speech was a milestone in the "Khrushchev Thaw."
More information, and a summary of the speech, on the Wikipedia page .