The Threshold of Space is a brief study of the national space program from 1954 through 1959 with emphasis on the role of the Air Force. It was originally prepared as a chapter for inclusion in the History of Headquarters USAF, Fiscal Year 1959. Because of the importance and timeliness of the subject, the chapter is being issued as a separate study to make it more quickly available throughout the Air Force. Based chiefly on official documents, The Threshold of Space is a precis of a much more detailed history on the space program currently being prepared. Rather than confining itself to fiscal year 1959, the present study reaches back to the beginnings of space research in the l940's and carries the story forward to January 1960. This was necessary to provide the proper perspective for an understanding and appreciation of this vital area of national activity. It was impossible in a study of this length to cover all facets of the space program, whether national or Air Force. There had to be a choice of topics such as policy, the selection of projects for development, and the widespread distribution by the Department of Defense of systems and subsystems among the three services for research and tests. Other topics almost equally important had to be excluded. Among the latter there were such subjects as interservice rivalry for control of the satellite-detection fence and the Navy-Air Force dispute about the Pacific Missile Range. It was also necessary to omit coverage of the valuable work done by the Air Force in the field of space medicine and in the establishment of international agreements for the construction of bases outside the United States. These and other subjects will receive thorough treatment in the more comprehensive history now under way.