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. The year 1964 marks the tenth anniversary of the inception of the X-15 flight-research program, the fifth year since the first X-15 flight. When the program was first approved, its objectives were clearly stated in terms of aerodynamic heating, speed, altitude, stability-and-control research, and bioastronautics. Although these objectives have been essentially accomplished, it now appears that the three X-15's may be flown for perhaps another five years, in a new role as test beds for fresh experiments utilizing the X-15 performance, which still offers more than twice the speed and three times the altitude capability of any other aircraft now in existence. Even though the program has been most successful in terms of achieving its planned objectives and is continuing to play an important role in aerospace research, many notable benefits have been of a different nature-more intangible and somewhat unforeseen at the time the X-15 program was approved. In the early years of our nation's space program, which has been based to a large extent on the unmanned-missile technology that had been developed over the five years prior to Project Mercury, the X-15 has kept in proper perspective the role of the pilot in future manned space programs. It has pointed the way to simplified operational concepts that should provide a high degree of redundancy and increased chance of success in these future missions. All of the people in industry and in government who have had to face the problems of design and of building the hardware and making it work have gained experience of great value to the more recent programs now reaching flight phase and to future aeronautical and space endeavors of this country. The X-15 program and Project Mercury have represented a parallel, two-pronged approach to solving some of the problems of manned space flight. While Mercury was demonstrating man's capability to function effectively in space, the X-15 was demonstrating man's ability to control a high-performance vehicle in a near-space environment. At the same time, considerable new knowledge was obtained on the techniques and problems associated with lifting reentry. Already the lessons learned are being applied to our new manned space programs. The pilot is playing a much greater role in these programs. Certainly the problem of launching the lunar-excursion module from the surface of the Moon through the sole efforts of its two-man crew must appear more practical and feasible in the light of the repeated launchings of the X-15 through the efforts of its pilot and the launch operator on the carrier B-52 than would be the case if it were compared only with the elaborate launch procedures and large numbers of people, buried safely in blockhouses, that typify all other launch operations to date. Future space programs may well include a lifting reentry and a more conventional landing on Earth, in the fashion demonstrated by the X-15.