NASA, and its predecessor, the National Advisory Committee for Aeronautics, have been conducting aeronautics research since 1915, helping make US aviation the multi-billion dollar industry it is today. But NASA is not alone, and is strongly committed to a thriving partnership with the Federal Aviation Administration is support of US aviation. NASA's Aeronautics program traditionally concentrated on improving a vehicle's efficiency and performance. However, a vehicle's ultimate performance is dictated by its ability to operate within an integrated aviation system. This must reflect the constraints of standards and regulations that ensure safety and a clean and quiet environment. It must also recognize a user's ability to operate productively in an airspace system that accommodates the users' need for flexibility within the system's capacity. Aeronautics' programs span all elements of this integrated aviation system. This paper will focus on those efforts to ensure user flexibility and system capacity, and provide an overview of NASA airspace operations and flight systems technology and its diverse customer base of technology products. The research and technology that we apply to today's garden of planned improvements for avionics and air traffic must be husbanded in the context of an integrated aviation system. In that way the aviation community will be able to harvest the next century's low-hanging fruit in real technologies and procedures. The operational concept called free flight is the keystone.