Sergio Leone (/liˈoʊni/ lee-OH-nee; Italian: [ˈsɛrdʒo leˈoːne]; 3 January 1929 – 30 April 1989) was an Italian filmmaker, credited as the pioneer of the spaghetti Western genre. He is widely regarded as one of the greatest directors in the history of cinema.

Leone's film-making style includes juxtaposing extreme close-up shots with lengthy long shots. His films include the Dollars Trilogy of Westerns featuring Clint Eastwood: A Fistful of Dollars (1964), For a Few Dollars More (1965), and The Good, the Bad and the Ugly (1966); and the Once Upon a Time films: Once Upon a Time in the West (1968), Duck, You Sucker! (1971), and Once Upon a Time in America (1984).

Born on 3 January 1929 in Rome, Leone was the son of the cinema pioneer Vincenzo Leone (known as Roberto Roberti or Leone Roberto Roberti) and silent film actress Edvige Valcarenghi (known as Bice Waleran). His mother was of Milanese and remote Austrian descent. During his schooldays, Leone was a classmate of his later musical collaborator Ennio Morricone in third grade. After watching his father work on film sets, Leone began his own career in the film industry at the age of 18 after dropping out of law studies at the university.

Working in Italian cinema, he began as an assistant to Vittorio De Sica during the production for the movie Bicycle Thieves in 1948. Leone began writing screenplays during the 1950s, primarily for the "sword and sandal" (or peplum) historical epics, popular at the time. He also worked as an assistant director on several large-scale international productions shot at the Cinecittà Studios in Rome, notably Quo Vadis (1951) and Ben-Hur (1959), financially backed by the American studios.

When director Mario Bonnard fell ill during the production of the 1959 Italian epic The Last Days of Pompeii (Gli Ultimi Giorni di Pompei), starring Steve Reeves, Leone was asked to step in and complete the film. As a result, when the time came to make his solo directorial debut with The Colossus of Rhodes (Il Colosso di Rodi, 1961), Leone was well-equipped to produce low-budget films that looked like larger-budget Hollywood movies.

Italian: Il cinema deve essere spettacolo, è questo che il pubblico vuole. E per me lo spettacolo più bello è quello del mito. Cinema must be spectacle, that's what the public wants. And for me the most beautiful spectacle is that of the myth.

In the mid-1960s, historical epics fell out of favor with audiences, but Leone had shifted his attention to a subgenre which came to be known as the "spaghetti Western", owing its origin to the American Western. His film A Fistful of Dollars (Per un pugno di dollari, 1964) was based upon Akira Kurosawa's Edo-era samurai adventure Yojimbo (1961). Leone's film elicited a legal challenge from the Japanese director, though Kurosawa's film was, in turn, probably based on the 1929 Dashiell Hammett novel, Red Harvest. A Fistful of Dollars is also notable for establishing Clint Eastwood as a star. Until that time, Eastwood had been an American television actor with few credited film roles.

The look of A Fistful of Dollars was established by its Spanish locations, which presented a violent and morally complex vision of the American Old West. The film paid tribute to traditional American Western films, but significantly departed from them in storyline, plot, characterization, and mood. Leone gains credit for one great breakthrough in the Western genre still followed today; in traditional Western films, many heroes and villains looked alike as if they had just stepped out of a fashion magazine, with clearly drawn moral opposites, even down to the hero wearing a white hat and the villain wearing a black hat (except for the most successful of the "traditional western cowboys" – Hopalong Cassidy, who wore a black outfit upon a pale horse). Leone's characters were, in contrast, more "realistic" and complex: usually lone wolves in their behavior; they rarely shaved, looked dirty, and sweated profusely, with a strong suggestion of criminal behavior. The characters were also morally ambiguous by appearing generously compassionate, or nakedly and brutally self-serving, as the situation demanded. Relationships revolved around power and retributions were emotion-driven rather than conscience-driven. Some critics have noted the irony of an Italian director who could not speak English, and had never even visited the United States, let alone the American Old West, almost single-handedly redefining the typical vision of the American cowboy. According to Christopher Frayling's book Something to do with Death, Leone knew a great deal about the American Old West. It fascinated him as a child, which carried into his adulthood and his films.