In a period of a little more than sixty years since the first flight of the Wright Brothers, man’s exploration of three-dimensional space above the surface of the Earth has extended beyond the atmosphere. Spectacular and exciting events in this dramatic quest have been well publicized. Behind these milestones of practical flight have been less publicized achievements in scientific research, making such progress possible. Although the X–15 has had its share of newsworthy milestones, its contributions to scientific research have been a more essential and more meaningful part of the program from its inception. This semi-technical summary of the X–15 program is directed toward the less publicized aspects of its achievements.