Space is used extensively for military purposes. There are over 170 military satellites in orbit, with uses ranging from navigation and communication to reconnaissance. In future, the military use of space might not be confined to satellite systems. The US plans to develop space based interceptors as part of National Missile Defence (NMD) and a number of recent US doctrinal statements indicate plans to develop space weapons systems. This POSTnote will review military uses of space worldwide. It will review US plans for space based missile defence and space weapons, discuss the technological challenges faced and examine the wider political implications. It will also review UK military activities in space, and will outline the status of ongoing discussions over the role of space in EU security. It will review current debate over whether using space for military purposes could jeopardize its accessibility and availability for civilian applications, and examine policy options to address this.