Asimov wrote or edited more than five hundred books and an estimated nine thousand letters and postcards, and has works in several major categories of the Dewey Decimal System. Asimov is widely considered a master of the science-fiction genre and, along with Robert A. Heinlein and Arthur C. Clarke, was considered one of the "Big Three" science-fiction writers during his lifetime. Asimov's most famous work is the Foundation Series; his other major series are the Galactic Empire series and the Robot series, both of which he later tied into the same fictional universe as the Foundation Series. He penned numerous short stories, among them "Nightfall", which was voted by the Science Fiction Writers of America the best of its kind up to 1964. He also wrote mysteries and fantasy, as well as a great amount of non-fiction. Asimov wrote the Lucky Starr series of juvenile science-fiction novels using the pen name Paul French.
Most of Asimov's popularized science books explain scientific concepts in a historical way, going as far back as possible to a time when the science in question was at its simplest stage. He often provides nationalities, birth dates, and death dates for the scientists he mentions, as well as etymologies and pronunciation guides for technical terms. Examples include his Guide to Science, the three volume set Understanding Physics, and Asimov's Chronology of Science and Discovery.
Asimov was a long-time member and Vice President of Mensa International, albeit reluctantly; he described the members of that organization as "intellectually combative". He took more joy in being President of the American Humanist Association. The asteroid 5020 Asimov, the magazine Asimov's Science Fiction, and two different Isaac Asimov Awards are named in his honor.