James Gunn--one of the founding figures of science fiction scholarship and teaching--wrote in 1951 what is likely the first master's thesis on modern science fiction. Portions appeared in the short-lived pulp magazine Dynamic but it has otherwise remained unavailable to the public. Here in its first full publication, the thesis explores many of the classic Golden Age stories of the 1940s and the critical perspective that informed Gunn's essential genre history Alternate Worlds and his anthology series The Road to Science Fiction. The editor's introduction and commentary show the historical significance of Gunn's work and its relevance to today's science fiction studies.