Today’s space systems fulfill five purposes: (1) environmental monitoring; (2) communications; (3) position, navigation, and timing; (4) integrated tactical warning and attack assessment; and (5) intelligence, surveillance, and reconnaissance missions. These missions are integral to a new American way of warfare. Direct and indirect challenges to American power in space are growing. Other nations are expanding their capabilities to interdict or deny U.S. access to space. Mounting fiscal pressures will likely necessitate changes in national “security space” force structures and acquisition approaches. This Special Report explores the implications of these challenges on U.S. national security space programs and policies. It sets the context for future decision making, providing insight into the myriad issues—from allied capability and intentions to extant arms control proposals—that will likely influence these decisions.The United States is approaching a critical juncture on its investments in national security space capabilities.