• 1.  The Complete Robot (1982).
  This is a collection of thirty-one robot stories published between 1940 and 1976 and includes every short story in the earlier collection I, Robot (1950).  As of 1988, only one robot short story had been written since this collection appeared.  That is "Robot Dreams," which appears in the recent short story collection, Robot Dreams.  (There may or may not be other robot stories written after 1988.)

The short story "Mother Earth" (1949) which appears in The Early Asimov chronologically takes place between the robot stories and book 2.

• 2.  The Caves of Steel (1954).
  This is the first of the robot novels.
• 3.  The Naked Sun (1957).
  The second robot novel.

The short story "Mirror Image" (1972) which appears in, among other places, The Complete Robot, chronologically takes place between books 3 and 4.

• 4.  The Robots of Dawn (1983).
  The third robot novel.
• 5.  Robots and Empire (1985).
  The fourth robot novel.

Isaac felt that there was room for an additional novel between 5 and 6, and hinted that he might have some day written one.

• 6.  The Stars, Like Dust - (1951).
  This is the first of the Empire novels.
• 7.  The Currents of Space (1952).
  The second Empire novel.
• 8.  Pebble in the Sky (1950).
  The third Empire novel.

The short story "Blind Alley" (1945) which appears in The Early Asimov chronologically takes place between books 8 and 9.  Contarary to popular belief, there are aliens in the Foundation universe, and it is this story that introduces them.

• 9.  Prelude to Foundation (1988).
  This is the first Foundation novel.
• 10. Forward the Foundation (1993).
  The second Foundation novel - and the last one written.  A collection of four novellas, three of which were originally published in Isaac Asimov's Science Fiction Magazine. Isaac did not have the opportunity to write the concluding fifth novella. Also contains an epilog of unknown authorship written especially for the book. (See the Isaac Asimov FAQ for more details.)
  [review/summary] [buy it]
• 11. Foundation (1951).
  The third Foundation novel - and the first one written.  Actually, it is a collection of four stories, originally published between 1942 and 1944, plus an introductory section written for the book in 1949.
• 12. Foundation and Empire (1952).
  The fourth Foundation novel, made up of two stories originally published in 1945.
• 13. Second Foundation (1953).
  The fifth Foundation novel, made up of two stories, originally published in 1948 and 1949.

11 - 13 are popularly known as The Foundation Trilogy.

• 14. Foundation's Edge (1982).
  The sixth Foundation novel.
• 15. Foundation and Earth (1986).
  The seventh Foundation novel.   [buy it]

Isaac intended to continue the series past Foundation and Earth, but did not reveal his ideas as to what direction a sequel would take.

Trevize looked up, as though to penetrate the thick layer of moonrock that separated him from the surface and from space; as though to force himself to see those far distant galaxies, moving slowly through unimaginable vistas of space.

He said, "In all human history, no other intelligence has impinged on us, to our knowledge.  This need only continue a few more centuries, perhaps little more than one ten thousandth of the time civilization has already existed, and we will be safe.  After all," and here Trevize felt a sudden twinge of trouble, which he forced himself to disregard, "it is not as though we had the enemy already here among us."

And he did not look down to meet the brooding eyes of Fallom - hermaphroditic, transductive, different - as they rested, unfathomably, on him.