I think we're going to have to cast aside our thoughts and memories of Asimov's original stories, and just try to enjoy it as a "
based upon characters created by Isaac Asimov" sort of deal.
To give some idea of how convoluted it could all get (thanks again to Wikipedia)
Merging with other series
The series is set in the same universe as Asimov's first published novel, Pebble in the Sky, although Foundation takes place about 10,000 years later. Pebble in the Sky became the basis for the Empire series. Then, at some unknown date (prior to writing Foundation's Edge) Asimov decided to merge the Foundation/Empire series with his Robot series. Thus, all three series are set in the same universe, giving them a combined length of 18 novels, and a total of about 1,500,000 words (see the List of books below). The merge also created a time-span of the series of around 20,000 years.
The stand-alone story Nemesis is also in the same continuity; being referenced in Forward the Foundation, where Hari Seldon refers to a twenty-thousand-year-old story of "a young woman that could communicate with an entire planet that circled a sun named Nemesis." Commentators noted that Nemesis contains barely disguised references to the Spacers and their calendar system, the Galactic Empire and even to Hari Seldon which seem to have been deliberately placed for the purpose of later integration into the Foundation universe.
Asimov's "Author's Note" in Prelude to Foundation
The foreword to Prelude to Foundation contains the chronological ordering of Asimov's science fiction books. Asimov stated that the books of his Robot, Empire, and Foundation series "offer a kind of history of the future, which is, perhaps, not completely consistent, since I did not plan consistency to begin with." Asimov also noted that the books in his list "were not written in the order in which (perhaps) they should be read".[47] In the Author's Note, Asimov noted that there is room for a book between Robots and Empire and The Currents of Space, and that he could follow Foundation and Earth with additional volumes.
Forward the Foundation, Nemesis, and The Positronic Man do not appear in Asimov's list, as they were not yet published at the time, and the order of the Empire novels in Asimov's list is not entirely consistent with other lists. For example, the 1983 Ballantine Books printing of The Robots of Dawn lists the Empire novels as: The Stars, Like Dust, The Currents of Space, and Pebble in the Sky. Given that The Currents of Space includes Trantor and that The Stars, Like Dust does not, these two books possibly were accidentally reversed in Asimov's list.
Asimov's standalone novels set in the Foundation universe
While not mentioned in the "Author's Note" of Prelude to Foundation, the novels The End of Eternity (1955), Nemesis (1989), and The Positronic Man (1992) are related to the greater Foundation series.
The End of Eternity is vaguely referenced in Foundation's Edge, where a character mentions the Eternals, whose "task it was to choose a reality that would be most suitable to Humanity". (The End of Eternity also refers to a "Galactic Empire" within its story.) Asimov himself did not mention The End of Eternity in the series listing from Prelude to Foundation. As for Nemesis, it was written after Prelude to Foundation, but in the author's note Asimov explicitly states that the book is not part of the Foundation or Empire series, but that some day he might tie it to the others.
In Forward the Foundation, Hari Seldon refers to a 20-thousand-year-old story of "a young woman that could communicate with an entire planet that circled a sun named Nemesis", a reference to Nemesis. In Nemesis, the main colony is one of the Fifty Settlements, a collection of orbital colonies that form a state. The Fifty Settlements possibly were the basis for the fifty Spacer worlds in the Robot stories. The implication at the end of Nemesis that the inhabitants of the off-Earth colonies are splitting off from Earthbound humans could also be connected to a similar implication about the Spacers in Mark W. Tiedemann's Robot books. According to Alasdair Wilkins, in a discussion posted on Gizmodo, "Asimov absolutely loves weird, elliptical structures. All three of his non-robot/Foundation science fiction novels — The End of Eternity, The Gods Themselves, and Nemesis — leaned heavily on non-chronological narratives, and he does it with gusto in The Gods Themselves."
In The Robots of Dawn, Dr. Han Fastolfe briefly summarizes the story from The Positronic Man (1992) or "The Bicentennial Man" (1976) in a conversation with Elijah Baley.
And finally...
Asimov's novels set in the greater Robot/Empire/Foundation universe
The following novels are listed in chronological order by narrative:
I, Robot (1950) - a fixup novel composed of 9 short stories about robots, set in the 21st century on Earth
The Positronic Man (1992) - a standalone robot novel, co-written with Robert Silverberg, based on Asimov's 1976 novelette "The Bicentennial Man", set from the 22nd to 24th centuries
Nemesis (1989) - a standalone novel, set in the 23rd century in a star system about 2 light years from Earth, when interstellar travel was new
The Caves of Steel (1954) - first Robot Series/R. Daneel Olivaw novel, set in the 35th century on Earth[49]
The Naked Sun (1957) - second Robot Series/R. Daneel Olivaw novel, set in the 35th century on the Spacer planet Solaria[49]
The Robots of Dawn (1983) - third Robot Series/R. Daneel Olivaw novel, set in the 35th century on the Spacer planet Aurora[49]
Robots and Empire (1985) - fourth Robot Series/R. Daneel Olivaw novel, set in the 37th century on Earth, Solaria, Aurora, and Baleyworld[49]
The Stars, Like Dust (1951) - first Empire Series novel, set thousands of years in the future before the founding of a Galactic Empire
The Currents of Space (1952) - second Empire Series novel, set thousands of years in the future during Trantor's unification of the galaxy into a Galactic Empire
Pebble in the Sky (1950) - third Empire Series novel, primarily set thousands of years in the future on Earth, when the galaxy is unified into a Galactic Empire
Prelude to Foundation (1988) - first Foundation Series novel
Forward the Foundation (1993) - second Foundation Series novel
Foundation (1951) - third Foundation Series novel
Foundation and Empire (1952) - fourth Foundation Series novel
Second Foundation (1953) - fifth Foundation Series novel
Foundation's Edge (1982) - sixth Foundation Series novel
Foundation and Earth (1986) - seventh Foundation Series novel
The End of Eternity (1955) - a standalone novel, about Eternity, an organization "outside time" which aims to improve human happiness by altering history
And that's without the books and stories that other authors have contributed to the canon.
I'll leave you with these two snippets:
According to lead singer Ian Gillan, the hard rock band Deep Purple's song The Mule is based on the Foundation character: "Yes, The Mule was inspired by Asimov. It's been a while but I'm sure you've made the right connection... Asimov was required reading in the 60's
Asimov himself commented that his fiction's internal history was "actually made up ad hoc. My cross-references in the novels are thrown in as they occur to me and did not come from a systemized history. ... If some reader checks my stories carefully and finds that my dating is internally inconsistent, I can only say I'm not surprised."
I look forward to having the same conversation over Dune.