Back to the Future is a 1985 American science fiction film directed by Robert Zemeckis, and written by Zemeckis and Bob Gale. It stars Michael J. Fox, Christopher Lloyd, Lea Thompson, Crispin Glover, and Thomas F. Wilson. Set in 1985, the story follows Marty McFly (Fox), a teenager accidentally sent back to 1955 in a time-traveling DeLorean automobile built by his eccentric scientist friend Doctor Emmett "Doc" Brown (Lloyd). Trapped in the past, Marty inadvertently prevents his future parents' meeting—threatening his existence—and is forced to reconcile the pair and somehow get back to the future.

Back to the Future was conceived of in 1980 by Gale and Zemeckis. They were desperate for a successful film after numerous collaborative failures, but their idea was rejected over 40 times by studios because it was not considered raunchy enough to compete with the successful comedies of the era. A development deal was secured following Zemeckis' success directing Romancing the Stone (1984). Fox was the first choice to portray Marty, but he was unavailable; Eric Stoltz was cast instead. Shortly after principal photography began in November 1984, Zemeckis determined Stoltz was not right for the part and made the concessions necessary to hire Fox. This included re-filming scenes already shot with Stoltz and adding $4 million to the budget. Back to the Future was filmed in and around California and on sets at Universal Studios. Filming concluded the following April.

Replacing Stoltz delayed production and the film's release date but, following highly successful test screenings, the date was brought forward to July 3, 1985, to give Back to the Future more time in theaters. This resulted in a rushed post-production schedule, and some incomplete special effects. Back to the Future was a critical and commercial success, earning $381.1 million to become the highest-grossing film of 1985 worldwide. Critics praised the story, comedy, and the cast—particularly Fox, Lloyd, Thompson, and Glover. It received multiple award nominations and won an Academy Award, Saturn Awards, and a Hugo Award. Its theme song, "The Power of Love" by Huey Lewis and the News, was a significant success globally.

In the years since its release, Back to the Future has grown in esteem and is now considered to be one of the greatest films of all time. In 2007, the United States Library of Congress selected the film for preservation in the National Film Registry. The film was followed by two sequels, Back to the Future Part II (1989) and Back to the Future Part III (1990). With its effect on popular culture and a dedicated fan following, Back to the Future launched a multimedia franchise. This includes an animated television series, video games, comic books, board games, clothing, music, books, food, toys, collectibles, and theme park rides. Its enduring popularity has led to numerous books about its production, documentaries, and commercials. Back to the Future has been adapted into a 2020 stage musical.