'X' Stands for Unknown is a collection of seventeen nonfiction science essays written by Isaac Asimov. It was the seventeenth of a series of books collecting essays from The Magazine of Fantasy and Science Fiction, these being first published between January 1982 and May 1983. It was first published by Doubleday & Company in 1984.

This article relies largely or entirely on a single source. (November 2020)

Quick Facts Author, Country ...

'X' Stands for Unknown

First edition
Author	Isaac Asimov
Country	United States
Language	English
Series	Fantasy & Science Fiction essays
Genre	science essay
Published	1984 (Doubleday)
Media type	print (Hardback and Paperback)
Pages	218
ISBN	0-385-18915-X
Preceded by	Counting the Eons
Followed by	The Subatomic Monster

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• Physics
  • 1 Read Out Your Good Book in Verse (May 1982)
  • 2 Four Hundred Octaves (June 1982)
  • 3 The Three Who Died Too Soon (July 1982)
  • 4 X Stands for Unknown (August 1982)
• Chemistry
  • 5 Big Brother (September 1982)
  • 6 Bread and Stone (October 1982)
  • 7 A Difference of an 'E' (November 1982)
  • 8 Silicon Life After All (December 1982)
• Astronomy
  • 9 The Long Ellipse (January 1982)
  • 10 Change of Time and State (April 1982)
  • 11 Whatzisname's Orbit (March 1982)
  • 12 Ready and Waiting (February 1983)
  • 13 Dead Centre (April 1983)
  • 14 Out in the Boondocks (May 1983)
• Mathematics
  • 15 To Ungild Refinèd Gold (January 1983)
  • 16 The Circle of the Earth (February 1982)
  • 17 The Armies of the Night (March 1983)

Dave Langford reviewed X Stands for Unknown for White Dwarf #79, and stated that "If you're scientifically literate you'll find the interesting bits buried in over-familiar stuff (though I always cheer Asimov when he stomps the crackpots). If not, you probably don't read books like this. That's showbiz."^[1]

• Review by Edward James (1985) in Paperback Inferno, #55^[2]