The Foundation series is a science fiction book series written by American author Isaac Asimov. First published as a series of short stories and novellas from 1942 to 1950, and subsequently in three collections, for nearly thirty years the series was a trilogy: Foundation (1951); Foundation and Empire (1952); and Second Foundation (1953). It won the one-time Hugo Award for "Best All-Time Series" in 1966. Asimov later added new volumes, with two sequels, Foundation's Edge (1982) and Foundation and Earth (1986), and two prequels, Prelude to Foundation (1988) and Forward the Foundation (1993).

The premise of the stories is that in the waning days of a still powerful Galactic Empire, the mathematician Hari Seldon spends his life developing a theory of psychohistory, a new and effective mathematics of sociology. Using statistical laws of mass action, it can predict the future of large populations. Seldon foresees the imminent fall of the Empire, which encompasses the entire Milky Way, and a Dark Age lasting 30,000 years before a second empire arises. Although the momentum of the Empire's fall is too great to stop, Seldon devises a plan by which "the onrushing mass of events must be deflected just a little" to eventually limit this interregnum to just one thousand years.

The plot of the Foundation series spans centuries, and its various characters each appear in one or two of its nine installments. Charles Elkins described its characters as "undifferentiated and one-dimensional" speaking with an "impoverished vocabulary". He wrote that their consciousness "shows absolutely no historical development and hence fails to evoke in the reader any feeling for the future universe they inhabit". Elkins argued that characterization in general is subordinated to the overall conception of Asimov's project. James E. Gunn wrote that though the series of lead characters Lathan Devers, Salvor Hardin, Limmar Ponyets and Hober Mallow "may seem interchangeable", they are "as differentiated as the personages in most histories."

Through the eyes of the characters the inevitability of the forces of history, made manifest in the Seldon Plan, is demonstrated to the reader repeatedly. Elkins sees the characters in Foundation not as "tragic heroes. They are nondescript pawns, unable to take their destiny into their own hands." Only those elite few characters who understand the Plan can be considered free, with the Mule through his non-human psychic powers as the only exception. But while Elkins attributes the Foundation series a sense of "pervading fatalism", Gunn and Nicolas David Gevers point out that the obstacles presented in Asimov's galactic history are overcome by active individual characters "through the initiative and competence which the Foundations nurture in their citizens". Donald E. Palumbo asserts that it is exactly the "flatness of character and setting" which permit the series "to be a masterpiece". The heroism and depth of individual characters is consciously taken back by Asimov for the true hero of the series to stand out: "the sublime history of humankind itself".

An eight-part radio adaptation of the original three novels, called The Foundation Trilogy, was broadcast on BBC Radio 4 in 1973.

In 2021, Apple TV+ premiered a television series adaptation of the novels, Foundation, created by David S. Goyer and Josh Friedman. In 2023, Asimov's daughter Robyn Asimov, an executive producer for the series, said:

I love the character development. That was not my father's strong suit, and not necessarily his interest per se. It was all about the storytelling, and he did that so well that it was okay that the characters were a bit flat. What David [S. Goyer] did was especially ... I love the Cleon story. He gave life to these characters, and it brought the story to another level. The story was great anyway, but I think if, if my father had lived to see this, I think he would have been very, very impressed. My father would have loved to have seen the characters come to life. That's something that was not in his wheelhouse per se. And I think this would have excited him.

In the Foundation and Empire (1952) story "The General", Imperial General Bel Riose of Siwenna coerces Ducem Barr to aid him in his persecution of the Foundation, which Riose hopes to destroy both as a perceived threat to the Galactic Empire and to further his own ambitions. Barr is Riose's best choice as an "expert" on the Foundation, his father Onum having met Foundation-aligned Master Trader Hober Mallow during the events of "The Merchant Princes" in Foundation (1951). Barr aligns himself with Lathan Devers, a Foundation trader who has let himself be captured by Riose to disrupt the general's operation from the inside. When Devers' machinations are exposed, Barr helps him escape further interrogation by knocking Riose unconscious and fleeing the planet with Devers in tow. They travel to the Imperial capital planet, Trantor, and plot to implicate Riose and Emperor Cleon II's Privy Secretary Ammel Brodrig in a nonexistent conspiracy to overthrow Cleon. Though Barr and Devers are caught by the Secret Police, they escape and later learn that Riose and Brodrig have been arrested for treason and executed.