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

'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.

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)

Reception

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]