Originally published in 1999, America’s Space Sentinels won the American Astronautical Society’s prestigious Eugene Emme Astronautical Literature Award and quickly established itself as the definitive book for understanding a crucial component of the US national defence capabilities. It focused on the emergence and evolution of the Air Force’s Defense Support Program (DSP) satellite system, which came on line in 1970 and continued to perform at a high level through the turn of this century and beyond. For this new edition, Jeffrey Richelson covers significant developments during the last dozen years relating to the deployment of these satellites, especially the struggles to develop and launch the follow-on Space-Based Infrared System (SBIRS), beginning in the late 1990s and continuing up to the present. The result is a book that remains the first and best source of information regarding these vital programs. As Richelson notes, SBIRS, like its aging but still functioning predecessor, has been designed primarily to provide instant early warning of missile launches from around the globe—particularly China, Russia, North Korea, Pakistan, India and Iran—through the infra-red sensors carried on each satellite. But the new system—beset by hardware, software, fiscal, and political problems—has only managed to move forward in fits and starts. While it has done so, the DSP system has continued to monitor the skies above the earth; two key ground stations in Australia and Germany have closed; nuclear powers Russia and the United States conferred extensively over the so-called Y2K problem (concerned that a computer malfunction might produce false alarms of a missile attack); and worries over potential launches from nations perceived as hostile to American interests have increased substantially.