A predestination paradox (also called causal loop, causality loop, and (less frequently) closed loop or closed time loop) is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveler is caught in a loop of events that "predestines" or "predates" them to travel back in time. Because of the possibility of influencing the past while time traveling, one way of explaining why history does not change is by saying that whatever has happened must happen. A time traveler attempting to alter the past in this model, intentionally or not, would only be fulfilling their role in creating history as we know it, not changing it, or that the time-traveler's personal knowledge of history already includes their future travels to their own experience of the past.

In layman's terms, it means this: the time traveler is in the past, which means they were in the past before. Therefore, their presence is vital to the future, and they do something that causes the future to occur in the same way that their knowledge of the future has already happened. It is very closely related to the ontological paradox and usually occurs at the same time.

The predestination paradox is controversial because it seemingly negates the "common sense" notion that we are responsible for our own destinies, and, with adequate knowledge and preparation, we can alter their courses. The predestination paradox says the exact opposite, leaving doubt in believers' minds if they really have free will.

Back to the Future uses several different forms of time travel, and primarily deals with the concept of history being altered, or alternate realities being created. However, several minor details deal with the predestination paradox. For example, in 1955, Marty McFly (Michael J. Fox) discovers that he is the one who inspired Goldie Wilson, his town's African American mayor in 1985, to run for office by accidentally informing Wilson of his future in 1955. Also, by playing "Johnny B. Goode" at the 1955 high school dance, Marty becomes responsible for Chuck Berry's rock and roll composing the very song that Marty would learn to play. His friendship with his future parents led his mother-to-be into thinking that Marty is a nice name, implying that Marty inspired his own naming. The two sequels to the movie deal with similar variation, as well as other paradoxes.