The End of Eternity is a 1955 science fiction novel by Isaac Asimov with mystery and thriller elements on the subjects of time travel and social engineering. Its ultimate premise is that of a causal loop, a type of temporal paradox in which events and their causes form a loop. The novel was shortlisted for the Hugo Award for Best Novel.

In The End of Eternity, members of a time-changing organization called Eternity, known as "Eternals", seek to ensure that the conditions that led to Eternity's founding occur as history says that they occurred. At the end of the novel, the protagonist Andrew Harlan is placed in a situation in which he must decide whether to allow the "circle" to close and Eternity to be founded, or to allow the opposite to happen and prevent Eternity from having ever existed.

Many years later, Asimov tied this novel into his broader Foundation Series by hinting in Foundation's Edge that it is set in a universe where Eternity had existed, but was destroyed by Eternals, leading to an all-human galaxy. In the last chapter of The End of Eternity, Noÿs mentions hopes of a Galactic Empire.

In the future, humanity uses time travel to create Eternity, an organization "outside time" that aimed to improve human happiness by observing human history and, after careful analysis, directly making small actions that cause "reality changes". It also established trade between the various centuries to help those most in need. Its members, known as "Eternals", prioritize the reduction of human suffering at the cost of a loss to technology, art, and other endeavors, which are prevented from existing as a consequence of reality changes. Those enlisted travel "upwhen" and "downwhen" and re-enter time in devices called "kettles". They are unable to travel to times before the 27th century, when the temporal field powering Eternity was established, the limit being known as the "downwhen terminus". Also, the future of humanity's fate is unknown since Earth is empty by the year 15 million (the 150,000th century, or the 15,000th millennium), but that is preceded by a period called the Hidden Centuries, or the Void Millennia, from the years 7 million to 15 million (the 70,000th to the 150,000th centuries or 7,000th to the 15,000th millennia) in which, for unknown reasons, they cannot access the world outside Eternity to learn more.

Andrew Harlan is an Eternal and an outstanding Technician, a specialist at implementing reality changes, who is fascinated by the Primitive times before the downwhen terminus. Senior Computer Laban Twissell, the Dean of the Allwhen Council, instructs Harlan to teach a newcomer, Brinsley Sheridan Cooper, about the Primitive. Meanwhile, Harlan is also tasked by Assistant Computer Finge to spend a week in the 482nd century. He stays with Noÿs Lambent, a non-Eternal member of the period's aristocracy and falls in love with her. However, he discovers that a reality change will affect the century, and wishing to preserve Noÿs as she is, he breaks Eternal law, removes her from time, and hides her in the empty sections of Eternity that exist in the Hidden Centuries.

Harlan later finds that the kettles will not travel to the time in which he hid Noÿs because of a mysterious block at the 100,000th century (the year 10 million). He confronts Finge with a weapon and accuses him of sabotaging matters out of jealousy, but Finge states that he reported Harlan's conduct and denies placing the block. Harlan is summoned to the council but is not reprimanded. He deduces that because his transgressions were ignored, he must be there to serve a larger purpose. Harlan confronts Twissell and explains that he has been teaching himself temporal mathematics and believes that its 23rd-century inventor, Vikkor Mallansohn, must have been helped in his discovery by someone from his future. He concludes that his current role is training Cooper to do so. Twissell confirms that and adds that unknown to Cooper, Mallansohn's secret memoirs show that Cooper is the famous inventor Mallansohn. That must be kept from Cooper so that Eternity will be founded as it historically was. Harlan blackmails Twissell by threatening to destroy Cooper's ignorance unless Noÿs is returned, but he is outwitted. Twissell locks him in the control room with all controls deactivated other than the lever to send Cooper back, which matches the memoirs' statement that to have been there was his role. Harlan, enraged, breaks open the controls and changes the power output, which causes Cooper to be sent back to an unknown time, estimated to be in the early 20th century.

Twissell is aghast, but as Eternity still exists, he theorizes that it is possible to undo Harlan's damage and send Cooper back correctly for his mission. They think that Cooper might try to communicate by using an advertisement in one of Harlan's Primitive magazines, which would stand out only to an Eternal. Harlan finds that a magazine from 1932 has changed and now shows an advertisement in the form of a mushroom cloud, which no human could have known of in 1932. The first letters in the ad's four lines, "All the Talk Of the Market", spell out ATOM. However, Harlan refuses to tell Twissell about the advertisement until they bring Noÿs back from the Hidden Centuries, but Twissell insists that the block that Harlan encountered is theoretically impossible. As the two travel far upwhen to get her, Twissell speculates that the Hidden Centuries are a time in which humans evolved into something greater, do not want to be meddled with, and so blocked off Eternity's access to time from the 70,000th to the 150,000th century. He supposes that when Harlan inhabited an Eternity outpost in the Hidden Centuries with Noÿs, those future humans may have worried that Eternity was beginning an invasion, which led them to retaliate with a kettle block at the 100,000th century to prevent any more encroachments. However, Harlan and Twissell pass the 100,000th century unhindered and find Noÿs. Harlan then agrees to travel downwhen and to bring back Cooper so that he can be sent to the correct time for his mission, but only if Noÿs comes with him.

On arrival in 1932, Harlan holds Noÿs at gunpoint and reveals that he suspects her of being from the Hidden Centuries and that he has brought her so that she could not harm Eternity. Noÿs acknowledges that she is from that time and explains that her people have also developed time travel, but their method shows many possible futures, rather than just the single future seen by Eternity. They learned that humans would have been the first species to spread into the galaxy, but in each future in which Eternity existed, safety was given a priority, and by the time that humans had reached the stars, other species predominated and prevented them. As a result, humanity would become depressed and gradually die out. Noÿs's mission was to make the minimum change to history to remedy that by preventing Eternity from ever being founded. There were multiple ways of achieving that, and she chose an approach in which she and Harlan were together. Noÿs gives Harlan the choice of killing her and preserving Eternity or letting her live and allowing a different future to arise. Harlan, remembering the unhealthy interpersonal relationships between the Eternals and the sociological damage that he has seen to be done to people whose original "homewhen" had ceased to exist, agrees with her. His resolution causes a reality change to occur, and the kettle disappears, indicating that Eternity now never happened. The book ends by stating that it was "the end of Eternity – and the beginning of Infinity".