"Punishment Without Crime" by Ray Bradbury, "Arena" by Fredric Brown, "The Leech" by Robert Sheckley, "Through Channels" by Richard Matheson, "Lost Memory" by Peter Phillips, "Memorial" by Theodore Sturgeon, "Prott" by Margaret St. Clair, "Flies" by Isaac Asimov, "The Microscopic Giants" by Paul Ernst, "The Other Inauguration" by Anthony Boucher, "Nightmare Brother" by Alan E. Nourse, "Pipeline to Pluto" by Murray Leinster, "Impostor" by Philip K. Dick, "They" by Robert A. Heinlein and "Let Me Live in a House" by Chad Oliver (Collected 1955):
Conklin was one of the kings of mid-to-late-20th-century science-fiction anthologies, primarily of the reprint variety. As one of the first editors to get a chance to present science fiction to the growing market for paperbacks, Conklin introduced a lot of readers to both early and contemporary science-fiction greats.
Conklin claims that this is the first general anthology to present the mixed genre of science-fiction horror stories, and I can't see any reason to dispute him. Several of the stories would go on to become acknowledged classics, with "Arena" supplying a plot for a similar Star Trek: TOS episode and Dick's "Impostor" being turned into a lousy movie with Gary Sinise.
Paranoia, always a major trope of science fiction, and especially American science fiction, dominates the proceedings in disturbing tales like "They" and "Let Me Live in a House", while various alien invasions and infiltrations occur in several other stories. Boucher -- better known as the early editor of The Magazine of Fantasy and Science Fiction -- contributes a 1953 tale of U.S. politics that wouldn't seem out-of-place if it were published now. All in all, a solid collection. Recommended.