United States air traffic system (USATS) providing both air navigation and traffic separation became a nationwide governmental service in 1936 after two decades of expanding private and public activity. Within fifty years, this system has grown into an extraordinary matrix of 600 airports and 300,000 miles of airways in continuous flux and motion as millions of people and mountains of freight (and air mail) are shepherded throughout the U.S.. It has been a remarkable development of a very large-scale, publicly owned technical system with quite different properties than the other systems discussed in this book. It is at once, more far-flung and complex, and less integrated and dependent upon technologies as a means of coordination. It has a different relationship to the national state. After a brief review of the dimensions of the USATS, we turn to these properties, suggest their importance for more general understanding of large-scale technical systems, and go into more detail in describing the extraordinary development of the USATS.