Natural rights can justify legal rights to control and dispose of those resources exclusively – that is, rights of ownership. Ownership is justified on moral grounds when it seems likely in practice to help people acquire and use resources more effectively than alternate regimes would – especially, a system in which resources were open for everyone’s access and use and people enjoyed them with usufructs. This chapter studies four core or paradigm cases in which ownership facilitates use enough to be legitimate. One (associated with Aristotle) stresses ownership’s tendency to reduce disputes over property; another (associated with St. Thomas Aquinas) focuses on how ownership encourages careful management of resources; a third (Locke) focuses on how ownership incentivizes people labor and productivity; and the last (James Madison and other American founders) focuses on ownership’s securing privacy and autonomy for owners’ own preferred uses. This chapter considers egalitarian critiques of ownership, especially by Jeremy Waldron, Joseph Singer, G.A. Cohen, Liam Murphy, and Thomas Nagel. To define ownership, this chapter relies on conceptual work by A.M. Honore and J.E. Penner.