The Holocaust "is one of the best documented events in history," and yet Holocaust denial was practiced while the Jews of Europe were being murdered in death camps during the second World War and has continued ever since.
The Nazis had an interest in denying they were attempting to exterminate European Jews. The Nazis actively attempted to destroy the evidence of the extermination camps in Belzec, Sobibor, and Treblinka, as well as several mass shooting sites across occupied Poland, Ukraine, and the Soviet Union.
Modern Holocaust denial has taken many forms. For a comprehensive list, please see the United States Holocaust Memorial's Key Dates in Holocaust Denial and the Antidefamation League's A Short History of Holocaust Denial in the United States.