This chapter explores the relation between sustainability and human space activities. While the sustainability framework can and perhaps ought to give guidance for discussing, planning, and implementing different human activities in space, the prospect of a long-term space future and humanity’s turn into a multiplanetary species is impactful for the concept of sustainability itself. Taking the standard definition of sustainability from the Brundtland Commission’s 1987 report Our Common Future as a starting point, the chapter will discuss the conceptual and normative effects of the New Space Age for three concepts or ideas that are key to sustainability’s definition. These concepts and ideas are substitutability, the idea that there are planetary limits to economic growth, and the notion of future generations. The chapter suggests that to remain relevant for the long-term future, sustainability must accommodate space activities, and cannot maintain its Earthist or Earth-bound conceptual ties to there being only one planet to inhabit, develop, and sustain life in.