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