Time travel is the hypothetical activity of traveling into the past or future. Time travel is a concept in philosophy and fiction, particularly science fiction. In fiction, time travel is typically achieved through the use of a device known as a time machine. The idea of a time machine was popularized by H. G. Wells's 1895 novel The Time Machine.

It is uncertain whether time travel to the past would be physically possible. Such travel, if at all feasible, may give rise to questions of causality. Forward time travel, outside the usual sense of the perception of time, is an extensively observed phenomenon and is well understood within the framework of special relativity and general relativity. However, making one body advance or delay more than a few milliseconds compared to another body is not feasible with current technology. As for backward time travel, it is possible to find solutions in general relativity that allow for it, such as a rotating black hole. Traveling to an arbitrary point in spacetime has very limited support in theoretical physics, and is usually connected only with quantum mechanics or wormholes.

Some ancient stories feature characters who appear to leap forward in time. Vishnu Purana, in Hindu mythology narrate the myth of Raivata Kakudmi, who visits the god Brahma in heaven and returns to Earth to find that many ages have passed. In the Buddhist Pāli Canon, the Payasi Sutta describes how the disciple Kumara Kassapa explains to a skeptic that time flows differently in the heavenly realms. The Japanese legend of "Urashima Tarō", first recorded in the Manyoshu, tells of a fisherman who visits an undersea palace for three days, only to return and find that centuries have passed and his world is gone.

In one tradition in Judaism, Moses is transported by God to the study hall of Rabbi Akiva, where he is confused by the future evolution of Jewish law. Another Talmudic story features Honi HaMe'agel, a 1st-century BCE miracle worker who sees a man planting a carob tree that will take 70 years to bear fruit. Honi falls asleep and awakens 70 years later to find the tree fully grown and its fruit being harvested by the man's grandson.

In Islam, the Quran narrates the story of the Seven Sleepers, a group of monotheistic young men who sought refuge in a cave to escape persecution. As they slept, Allah preserved them for centuries, and when they awoke, they discovered that the world around them had changed. This narrative, found in the Quranic Surah Al-Kahf, describes divine protection and time suspension.

Time travel themes in science fiction and the media can be grouped into three categories: immutable timeline; mutable timeline; and alternate histories, as in the interacting-many-worlds interpretation. The non-scientific term 'timeline' is often used to refer to all physical events in history, so that where events are changed, the time traveler is described as creating a new timeline.

Early science fiction stories feature characters who sleep for years and awaken in a changed society, or are transported to the past through supernatural means. Among them L'An 2440, rêve s'il en fût jamais (The Year 2440: A Dream If Ever There Was One, 1770) by Louis-Sébastien Mercier, Rip Van Winkle (1819) by Washington Irving, Looking Backward (1888) by Edward Bellamy, and When the Sleeper Awakes (1899) by H. G. Wells. Prolonged sleep is used as a means of time travel in these stories.

The date of the earliest work about backwards time travel is uncertain. The Chinese novel A Supplement to the Journey to the West (c. 1640) by Dong Yue features magical mirrors and jade gateways that connect various points in time. The protagonist Sun Wukong travels back in time to the "World of the Ancients" (Qin dynasty) to retrieve a magical bell and then travels forward to the "World of the Future" (Song dynasty) to find an emperor who has been exiled in time. However, the time travel is taking place inside an illusory dream world created by the villain to distract and entrap him. Samuel Madden's Memoirs of the Twentieth Century (1733) is a series of letters from British ambassadors in 1997 and 1998 to diplomats in the past, conveying the political and religious conditions of the future. Because the narrator receives these letters from his guardian angel, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel". Madden does not explain how the angel obtains these documents, but Alkon asserts that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backward from the future to be discovered in the present". In the science fiction anthology Far Boundaries (1951), editor August Derleth claims that an early short story about time travel is An Anachronism; or, Missing One's Coach, written for the Dublin Literary Magazine by an anonymous author in the June 1838 issue. While the narrator waits under a tree for a coach to take him out of Newcastle upon Tyne, he is transported back in time over a thousand years. He encounters the Venerable Bede in a monastery and explains to him the developments of the coming centuries. However, the story never makes it clear whether these events are real or a dream. Another early work about time travel is The Forebears of Kalimeros: Alexander, son of Philip of Macedon by Alexander Veltman published in 1836.