A well-established nuclear/radiological incident emergency management is one of the essential aspects of U.S. nuclear capacity. After over 60-year-long development and reform, the four tier system has evolved one in which the federal agencies are taking foremost roles, combining the vertical line of leadership with state and local governments. The whole system is based on laws and specific regulations, specialized agencies, rich dedicated assets, comparatively good preparedness and capabilities of effective emergency responses. Many aspects of it are the most advanced in the world. With the American distinctiveness, the process of its building-up not only illustrates the close relationship between nuclear security and national security, but also reflects the professional requirements of nuclear/radiological crisis management. Nowadays the system has become an integrated part of federal disasters emergency management, and it is playing a significant role in defending the public safety as well as homeland security of the United States. <<