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Performance by an Actor in a Leading Role is one of the Academy Awards of Merit presented annually by the Academy of Motion Picture Arts and Sciences (AMPAS) to recognize an actor who has delivered an outstanding performance while working within the film industry. Prior to the 49th Academy Awards ceremony (1976), this award was simply known as the Academy Award of Merit for Performance by an Actor. Since its inception, however, the award has commonly been referred to as the Oscar for Best Actor. While actors are nominated for this award by Academy members who are actors and actresses themselves, winners are selected by the Academy membership as a whole.

Throughout the past 85 years, accounting for ties and repeat winners, AMPAS has presented a total of 86 Best Actor awards to 76 different actors. Winners of this Academy Award of Merit receive the familiar Oscar statuette, depicting a gold-plated knight holding a crusader's sword and standing on a reel of film. The first recipient was Emil Jannings, who was honored at the 1st Academy Awards ceremony (1929) for his performances in The Last Command and The Way of All Flesh. The most recent recipient was Daniel Day-Lewis, who was honored at the 85th Academy Awards ceremony (2012) for his performance in Lincoln.

In the first three years of the Academy Awards, individuals such as actors and directors were nominated as the best in their categories. At that time, all of their work during the qualifying period (as many as three films, in some cases) was listed after the award. However, during the 3rd Academy Awards ceremony (1930), only one of those films was cited in each winner's final award, even though each of the acting winners had had two films following their names on the ballots. For the 4th Academy Awards ceremony (1931), this unwieldy and confusing system was replaced by the current system in which an actor is nominated for a specific performance in a single film. Such nominations are limited to five per year. Until the 8th Academy Awards ceremony (1936), nominations for the Best Actor award were intended to include all actors, whether the performance was in either a leading or supporting role. At the 9th Academy Awards ceremony (1937), however, the Best Supporting Actor category was specifically introduced as a distinct award following complaints that the single Best Actor category necessarily favored leading performers with the most screen time. Nonetheless, Lionel Barrymore had received a Best Actor award (A Free Soul, 1931) and Franchot Tone a Best Actor nomination (Mutiny on the Bounty, 1936) for their performances in what were clearly supporting roles. Currently, Performance by an Actor in a Leading Role, Performance by an Actress in a Leading Role, Performance by an Actor in a Supporting Role, and Performance by an Actress in a Supporting Role constitute the four Academy Awards of Merit for acting annually presented by AMPAS.

Actors have also received special awards, or Academy Honorary Awards, for acting in specific films (such as in the case of James Baskett, who received a special honorary award for Disney's Song of the South). Child actors have also been awarded the Academy Juvenile Award.

Daniel Day-Lewis is the only three-time winner for the Best Actor award as well as the only non-American actor to have won more than one award. Eight other men have won the Best Actor award twice. In chronological order, they are: Spencer Tracy (1937, 1938), Fredric March (1932, 1946), Gary Cooper (1941, 1952), Marlon Brando (1954, 1972), Dustin Hoffman (1979, 1988), Tom Hanks (1993, 1994), Jack Nicholson (1975, 1997), and Sean Penn (2003, 2008). Tracy and Hanks were the only actors to win their awards in consecutive years. Furthermore, Tracy and Hanks were the same age at the time they received their Academy Awards: 37 for the first and 38 for the second.