The first was a digest-sized, bi-monthly magazine, published from 1952 to 1954 by Science Fiction Publications (later Future Publications), New York, for a total run of nine issues. Lester del Rey was editor (under his Philip St. John pseudonym) for Volume 1 (1952-1953), followed by Harry Harrison for Volume 2.
The second magazine with this title was also digest-sized and bi-monthly (initially), published from 1956 to 1958 by Royal Publications, New York, for a total run of 12 issues. Larry Shaw was editor. This magazine was published as a companion to Infinity Science Fiction.