In this chapter, students will learn about the importance of outer space as a strategic domain, for national and international security. Space has become a security domain of equal importance alongside land, sea, air, and cyberspace, because space-based technologies support military activities on Earth and have become so critical to military operations that it has become a contested strategic domain. But outer space is also a domain which forms part of our environmental, economic, and human security. This chapter examines the ways in which we are dependent on space, why a potential conflict extending into space would seriously impact us all, efforts to prevent a conflict and restrain the weaponization of space, as well as the disruptive importance of commercial actors which have now become integral to space security.