The Peaceful Uses of Outer Space is one of the most important, most contested, and most misunderstood norms of space law.  It is almost universally recognized as a feature of international space law, but at the same time its meaning as a constraint or obligation lacks clarity.  The principle that space should be used for peaceful purposes is rooted in the customary international law of space.  It is repeated in nearly every UN General Assembly resolution on space, finds itself in the name of the UN body dedicated to space activities (UNCOPUOS), and is a constant feature of diplomatic statements and texts regarding outer space.  As such, it constitutes a threshold for the legality of space activities and a grundnorm for space law.   Unfortunately, the content of this pervasive norm is contested in the international community and subject to change as states vie for interpretive authority and legitimacy in the space domain.  Peaceful Purposes, as a legal norm, finds itself stretched between concepts such a demilitarized and de-weaponized and among adjacent legal norms such as the ban on the use of force, the principle of nonaggression, and the right to self-defense.  In the wider arena of international law, the concept of ‘peace’ is relative to the concept of ‘defense,’ which has led some scholars to argue that Peaceful Purposes means nothing more than non-aggressive.  Yet, at the same time, there seems to be specific usage and employment of the concept in the space regime, which has led other scholars to argue that concept must have more normative content than the baselines extant in the general law concerning the use of force.   Part of the issue that leads to this problem is the customary nature of Peaceful Purposes.  The drafters of the Outer Space Treaty included it in the preamble of the Outer Space Treaty as an umbrella concept for space, but only used it once in the binding treaty text in Article IV.  In Article IV, the Moon and other celestial bodies are reserved “exclusively for peaceful purposes,” and – critically – this article includes specific limitations on the military uses of celestial bodies.  The word “exclusively” in this clause only furthers the ambiguity of our understanding of the broader norm as applied to space.  If space is for Peaceful Purposes but not exclusively so, then what has been barred and what is allowed?   The contested nature of this term should not surprise us. The UN Charter’s ban on the use of force sits at the heart of the contemporary system of international law, and is itself highly contested as debates on collective security, the nature of self-defense, and humanitarian intervention have shown us.  Peaceful Purposes’ adjacency to this legal norm means that it has become a locale of contestation as well. Core principles in legal systems are often battlegrounds for understanding the nature of the system.  Peaceful Purposes is no different.  It sits at the heart of space law, and as such will continue to evolve as states vie to interpret it and bend it to their desires.