The Foundation series is a science fiction series by Isaac Asimov. Seven volumes are in the Foundation series proper, which in its in-universe chronological order are Prelude to Foundation, Forward the Foundation, Foundation, Foundation and Empire, Second Foundation, Foundation's Edge, and Foundation and Earth.
The premise of the series is that the mathematician Hari Seldon spent his life developing a branch of mathematics known as psychohistory, a concept of mathematical sociology (analogous to mathematical physics). Using the laws of mass action, it can predict the future, but only on a large scale; it is error-prone on a small scale. It works on the principle that the behaviour of a mass of people is predictable if the quantity of this mass is very large (equal to the population of the galaxy, which has a population of quadrillions of humans, inhabiting millions of star systems). The larger the number, the more predictable is the future.
Using these techniques, Seldon foresees the imminent fall of the Galactic Empire, which encompasses the entire Milky Way, and a dark age lasting 30 thousand years before a second great empire arises. Seldon's psychohistory also foresees an alternative where the intermittent period will last only one thousand years. To ensure his vision of a second great empire comes to fruition, Seldon creates two foundations—small, secluded havens of all human knowledge—at "opposite ends of the galaxy".
The focus of the series is on the First Foundation and its attempts to overcome various obstacles during the formation and installation of the Second Empire. All the while (and often unknown to its major actors), it is being silently guided by the unknown specifics of the Seldon plan.
The series is best known for the Foundation Trilogy, which comprises the books Foundation, Foundation and Empire, and Second Foundation. While the term "Foundation series" can be used specifically for the seven Foundation books, it is also used more generally to include the Robot series (four novels) and Empire series (three novels). These seven books are set in the same fictional universe as the initial seven, but in earlier time periods. If all works are included, in total, 14 novels, as well as dozens of short stories, were written by Asimov. Also, seven novels were written by other authors after Asimov's death, which expand the time spanned in the original trilogy (roughly 550 years), by more than 20 thousand years. The series is highly acclaimed, and the Foundation Trilogy won the one-time Hugo Award for "Best All-Time Series" in 1966.