M is a 1931 film noir directed by Fritz Lang in which a serial killer, played by Peter Lorre, preys on children; the police and criminal underground both work to stop him. M was the first starring role for Peter Lorre, and it boosted his career, even though he was typecast as a villain for years after.

The tune Peter Lorre was whistling was "In the Hall of the Mountain King" from Peer Gynt by Edvard Grieg.

The film was based in part on the stories of Jack the Ripper and the Vampire of Düsseldorf and is consistently in the top 50 of the Internet Movie Database's top 250 films.

The movie was remade in 1951, failing to leave a mark on audiences.