William J. Monahan is an American screenwriter and novelist. His second produced screenplay was The Departed, a film that earned him a Writers Guild of America Award and Academy Award for Best Adapted Screenplay.

Infernal Affairs is a 2002 Hong Kong action thriller film co-directed by Andrew Lau and Alan Mak. Jointly written by Mak and Felix Chong, it stars Andy Lau, Tony Leung, Anthony Wong, Eric Tsang, Sammi Cheng and Kelly Chen. The film follows an undercover Hong Kong Police Force officer who infiltrates a Triad, and another officer who is secretly a spy for the same Triad. It is the first in the Infernal Affairs series and is followed by Infernal Affairs II and Infernal Affairs III.