The first Foundation Trilogy was originally published between 1951 and 1953, during the Golden Age of Science Fiction.  "Foundation," "Foundation And Empire," and "Second Foundation" are considered by many to be the greatest science fiction books ever written.  For the uninitiated, the concept is roughly equivalent to the historical fall of the Roman Empire, the subsequent Dark Ages that followed its collapse, and the rebirth of civilization during the Renaissance.  Of course, instead of the Roman Empire, which merely surrounded the Mediterranean Sea, Asimov imagined an Empire spanning across the entire galaxy.  Today this is a cliched idea to be sure, but in the nineteen fifties it was a brand new concept, one which would be copied by everyone from Frank Herbert to George Lucas.

The premise of the series is that scientist Hari Seldon spent his life developing a branch of mathematics known as psychohistory. Using the law of mass action, it can predict the future, but only on a large scale; it's error-prone for anything smaller than a planet or an empire. It works on the principle that the behaviour of a mass of people is predictable. And the quantity of this mass tends to be very huge (equal to the population of the galaxy). The larger the mass the more predictable is the future. Using these techniques, Seldon foresees the fall of the Galactic Empire, which encompasses the entire Milky Way, and a dark age lasting thirty thousand years before a second great empire arises. To shorten the period of barbarism, he creates two Foundations, small secluded havens of art and science, and all human knowledge on opposite ends of the galaxy.  The story follows the First Foundation, which Seldon establishes on the planet Terminus, the location of the other Foundation remains unknown.  Throughout the book, the Foundations must survive petty warlords, internal strife, a renewal of the Empire, plots against each other, and a mysterious mutant who threatens the entire plan.

The book is written in Asimov's trademark style. Little violence, even less sex, but a great plot and lots of cool technology. Like most of Asimov's stories, the characters are "cardboard"; these are fundamentally novels of ideas rather than action or character (ironically the later novels have more complex characters, but the ideas aren't as good).

A less common criticism of the Foundation series is it's unbelievability.  This is partly due to having been written in the fifties.  Even still, the technology he describes in the novels has not aged very well.  In Foundation, he describes the independent systems on the periphery of the galaxy having gone back to "coal and oil" for fuel, while the Foundation still uses atomic power.  Yet these worlds still have the ability to travel between different star systems.  As another example, in one scene in Foundation and Empire, a character gets the news by buying a paper at a vending machine.