'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.

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.