Inspired by Hugo Grotius, Mathias Risse has recently argued forcefully that any property or territorial arrangement that fails to provide the global population with an equal opportunity to satisfy basic needs is unjust: it violates natural rights of common ownership. This chapter argues that Grotius and Risse theorize Collective Ownership of the Earth in rival ways. Risse develops a theory of justice: COE generates moral rights that restrain the acceptability of exclusionary regimes. Grotius, by contrast, advances a theory of property: what shape should we give to conventionally instituted individual property rights, given that the earth originally belongs to everyone? This chapter focuses on the right of necessity to illustrate the differences between these approaches. Grotius conceptualizes retained rights of common ownership, including rights of necessity, as partly definitive of individual property and territorial rights. These retained rights therefore bind each and every property-holder. Risse declares any pattern of holdings that fails to realize natural rights of common ownership ‘unjust’, while insisting that the duty to bring about just distributions is incumbent upon states alone. I conclude that Grotius’s model resolves theoretically more parsimoniously the question of who are bound by the obligations corresponding to rights of common ownership.