Ace Books is a publisher of science fiction (SF) and fantasy books founded in New York City in 1952 by Aaron A. Wyn. It began as a genre publisher of mysteries and westerns, and soon branched out into other genres, publishing its first science fiction title in 1953. This was successful, and science fiction titles outnumbered both mysteries and westerns within a few years. Other genres also made an appearance, including nonfiction, gothic novels, media tie-in novelizations, and romances. Ace became known for the tête-bêche binding format used for many of its early books, although it did not originate the format. Most of the early titles were published in this "Ace Double" format, and Ace continued to issue books in varied genres, bound tête-bêche, until 1973.

Ace, along with Ballantine Books, was one of the leading science fiction publishers for its first ten years of operation. The death of owner A. A. Wyn in 1967 set the stage for a later decline in the publisher's fortunes. Two leading editors, Donald A. Wollheim and Terry Carr, left in 1971, and in 1972 Ace was sold to Grosset & Dunlap. Despite financial troubles, there were further successes, particularly with the third Ace Science Fiction Specials series, for which Carr came back as editor. Further mergers and acquisitions resulted in the company becoming absorbed by Berkley Books. Ace later became an imprint of Penguin Group (USA).