Monster is a 2003 American movie about the life of prostitute-turned-serial killer Aileen Wuornos, who was executed in 2002 for killing seven men in the late 1980s-early 1990s. The role of Aileen Wuornos was played by South African actress Charlize Theron, and her lesbian lover, Selby Walls (Tyria Moore in real life) was played by Christina Ricci. The film was written and directed by Patty Jenkins.

Much of Theron's preparation for the role is credited to Nick Broomfield's 1992 documentary Aileen Wuornos: The Selling of a Serial Killer, which she reportedly watched clips of between takes.

Tagline: Based on a true story

The main characters include:

Aileen - (Charlize Theron) a prostitute from Michigan who moved to Florida.
Selby - (Christina Ricci) A lesbian who becomes Aileen's girlfriend; she's based on Aileen's real-life girlfriend Tyria Moore.
Thomas - (Bruce Dern) An old barfly friend of Aileen's confides in occasionally.

After moving to Florida, Aileen meets Selby in a bar. Selby likes Aileen almost immediately despite the fact that Aileen is clearly a severely disturbed woman. They have a small encounter at a roller rink and quickly become committed to one another. Selby becomes so committed to Aileen that she and Aileen move into a small apartment together. It becomes hard to pay the bills, so Aileen tries to get a legitimate job. This is not easy for Aileen since she has no qualifications and has a very bad temper which she often displays during job interviews. Desperate for money, stressed and resentful of her clients, Aileen commits several murders, each more awful than the last. Toward the end of the movie, the once naïve Selby cooperates with the police to turn Aileen in. Aileen is arrested, jailed, and sentenced to death.

A large majority of film critics recommended this film. Most critics gave especially high praise to Theron's performance. Until her performance in Monster, Theron was best known for conventional supporting roles that a woman of her looks typically gets (such as in The Italian Job and The Cider House Rules), so it can be said that her performance as the unattractive, mentally ill, tragic Aileen was unexpected. Many critics called her performance (and her makeup in the film) a "transformation"

The film won Theron the Academy Award for Best Actress, Golden Globe Award for Best Actress in a Drama and the SAG Award.