This chapter studies the elements of an interest-based natural property right. To acquire a prima facie right in a resource, the claimant must use it productively and claim exclusivity to its use in terms others will understand. But the prima facie right may be overridden by either of two provisos. The sufficiency proviso limits property rights when a proprietor’s use of a resource does not leave others sufficient access to the same type of resource for their own needs. The necessity proviso limits natural rights when someone who does not hold property in a resource needs access to it to repel some serious threat to life or property. This chapter illustrates legal doctrines for capturing animals and other articles of personal property, occupying unowned land, and appropriating water flow by use. This chapter contrasts productive use with Locke’s treatments of labor, waste, and spoliation, and it contrasts claim communication with Pufendorf and Grotius’s treatments of possession. This chapter also considers familiar criticisms of rights-based property theories, involving hypotheticals with radioactive tomato juice or ham sandwiches embedded in cement.