This chapter explores how space technology affects strategic thought in the early twenty-first century. It discusses the spacecraft and related technology that have proven most useful in military operations and other strategic activities. The chapter outlines what currently existing space systems can and cannot do. It also discusses issues space technology raises for strategists. The limits to what spacecraft can do are less obvious. Nevertheless, most space vehicles perform only a few specific tasks. Armed forces that lack satellites of their own remain capable of besting space powers in war. Like communications satellites, satellite navigation systems improve on capabilities that armed forces already possessed. Under most circumstances, the most practical way to stop opponents from using space-based positioning systems is to jam transmissions from satellites. Space-based observation helps all parties assure themselves that their opponents are abiding by such agreements without requiring any country to accept visits from human inspectors or overflights by air-breathing aircraft.