- Mr. Strickland: "No McFly has ever amounted to anything in the history of Hill Valley!"
- Marty: "Yeah? Well, history is gonna change."
- — Strickland to Marty
Foreshadowing is a literary device in which subtle hints are dropped about plot developments that will come later in a story.
In the Back to the Future trilogy, some of the information conveyed through the device of exposition could be considered to be both information about past events, and at the same time foreshadowing of events that Marty will see after he travels back into the past.
Among the notable examples of foreshadowing in Back to the Future are:
- At the start of the film, we see a representation of actor Harold Lloyd hanging from the hands of a clock, from the film Safety Last. This foreshadows Emmett Brown hanging from the clock tower on November 12, 1955.
- Marty's skateboard rolls to a stop at a case of plutonium after a morning news broadcast reports that a Libyan terrorist group claimed responsibility for its theft.
- Marty takes hold of a Jeep as he skateboards through downtown Hill Valley in 1985 on his way to school. His skills come in handy in 1955 when he grabs the back of a pickup truck to skateboard away from Biff's gang.
- Hill Valley High School principal Stanford S. Strickland tells Marty that if he keeps hanging around Doc, "You're gonna end up in big trouble!"
- Marty comments "Well, history is gonna change" — not realizing that he will alter the present timeline.
- The McFly family is watching the episode of The Honeymooners where "Ralph dresses up as a man from space". This foreshadows Marty being mistaken for a "man from space" in 1955 (by the Peabody family and by George McFly).
- At the Twin Pines Mall, Doc mentions Otis Peabody, who owned the land where the mall stands, and who had a crazy idea for breeding pines. This seemingly irrelevant information sets up Marty's tree-killing dash around the Twin Pines Ranch; the payoff comes when Marty returns to the Lone Pine Mall at the film's end.
- At the Battle of the Bands in 1985, Marty is stopped less than a minute into his guitar performance, and laments later to Jennifer Parker that he'll never get the chance to play in front of anybody. This presages the moment later where fate gives him the chance to show off his talents at the Enchantment Under the Sea dance in 1955.
- Lost in thought in 1955, Marty is almost hit by a car as he wanders across the street in Courthouse Square. Without thinking, he ends up getting hit by his grandfather's car after pushing George out of the way.
- At the beginning of the film, Biff Tannen witnesses the DeLorean time machine depart for 2015 and shows that he still has a dark side as he mutters, "What the hell is going on here?" and establishing, not very subtly, that he will be the villain as the film progresses.
- The cashier at the Blast from the Past Antique memorabilia store explains how Grays Sports Almanac has a dust jacket that covers the book; the dust jacket becomes relevant later in the film.
- Doc is afraid to step in when the police pick up Jennifer in 2015, because he's afraid that "they'll have us committed". When he returns to 1985A, his worst fears have come true as he finds a newspaper with the headline EMMETT BROWN COMMITTED.
- In the Biff Tannen Museum documentary, one line is "You'll learn the amazing history of the Tannen family, starting with his great-grandfather, Buford "Mad Dog" Tannen, the fastest gun in the west" who is seen in Part III.
- Doc comments to Marty "My only regret is that I'll never get a chance to visit my favorite historical era, the Old West"; later, when he prepares to go to 1955, the time display malfunctions and changes to "JAN 01 1885 12:00 A.M." for a moment. Doc comments "Damn! I gotta fix that thing!" Though it isn't clear at that moment how a trip to the Old West will take place, it's certain that that will be in the future (or in the past).
- In 2015, Doc can be seen wearing a colorful shirt with numerous cowboys and steam trains on it. This may be a foreshadowing of the locomotive hijacking in 1885 to allow Marty to return to 1985.