What condition prevents Memento's Leonard Shelby from forming new memories?

Anterograde amnesia prevents Memento's Leonard Shelby from forming new long-term memories. After the attack that killed his wife, Leonard can remember much of his earlier life but struggles to retain events that happen afterward.

The condition drives both the plot and the film's unusual structure. Leonard compensates with Polaroid photographs, handwritten notes, and tattoos that preserve facts he expects to forget. The audience experiences a similar uncertainty because many color scenes are shown in reverse chronological order.

Anterograde amnesia is often confused with retrograde amnesia. Retrograde amnesia involves losing memories formed before an injury or illness; anterograde amnesia concerns the creation or storage of new memories after the event. Leonard's problem is therefore more specific than simply having “short-term memory loss.”

Memento also uses the fictional case of Sammy Jankis to explore memory, learning, and self-deception. Leonard's recollections are not always reliable, however, and the film suggests that his system of notes and tattoos can be manipulated or interpreted through his existing beliefs. The condition is a medical premise, while the film's fragmented storytelling turns it into a narrative experience.