Monster is a 2003 crime drama film about serial killer Aileen Wuornos, a former prostitute who was executed in Florida in 2002 for killing six men (she was not tried for a seventh murder) in the late 1980s and early 1990s. Wuornos was played by Charlize Theron, and her fictionalized lover, Selby Wall (based on Wuornos' reallife companion Tyria Moore), was played by Christina Ricci. The film was written and directed by Patty Jenkins.

Model-turned-actress Charlize Theron leaves her glamorous image behind for this gritty drama, in which she plays a disturbed prostitute who becomes a serial killer. Aileen Wuornos (Theron) was a woman who survived a brutal and abusive childhood in Michigan to become a thick-skinned but emotionally damaged adult. Homeless most of her life, Wuornos subsisted by working as a street prostitute; later, when she was in Florida, down to her last five dollars and pondering suicide, she stopped into a bar for a beer. There, Aileen met Selby Wall (Christina Ricci), a woman in her early twenties who had been sent to live with relatives after her Christian parents became aware of her lesbian lifestyle. Selby is immediately attracted to Aileen, and while Aileen tells Selby she's never been in a lesbian relationship, she soon finds herself equally infatuated with her. Selby runs away from her family and moves into a cheap hotel with Aileen, who initially pays the bills by hooking. However, as their money runs low and Aileen finds herself unable to land a regular job, tensions mount between the two. One night, after a john attacks her, Aileen pulls a gun and kills the man. Although her first murder can be categorized as self-defense, Aileen's loathing for the men who pay her for sex becomes so extreme that she begins killing her customers regardless of their behavior. Meanwhile, Selby slowly becomes aware of the full extent of her lover's instability and the bloody consequences of her actions. Monster was inspired by the true story of Aileen Wuornos, whose life and death was chronicled in two documentaries by filmmaker Nick Broomfield, Aileen Wuornos: The Selling of a Serial Killer, and Aileen: The Life and Death of a Serial Killer.

Film critics praised Monster; most gave overwhelmingly high praise to Theron's performance as an unattractive, mentally ill woman - Wuornos had antisocial personality disorder and borderline personality disorder. For the role, Theron gained 30 pounds and wore prosthetic teeth. Critics called her performance, and her makeup, a "transformation". Film critic Roger Ebert named it best film of the year, and wrote "What Charlize Theron achieves in Patty Jenkins'. 'Monster' isn't a performance but an embodiment... It is one of the greatest performances in the history of the cinema.

The film won Theron the Academy Award for Best Actress, Golden Globe Award for Best Actress in a Drama and the SAG Award. In 2009, Roger Ebert named it one of the best films of the decade.