Property is a deceptively familiar concept. The social world is structured in part by different laws, informal norms, and unspoken understandings about which things belong to which people and what you can and cannot do because of it. Yet the set of things that can be owned and the kinds of property rights recognized by the legal systems of the world are often very different from the paradigmatic forms of property we are all familiar with. Intellectual property rights--easily one of the most economically important categories of property--fit this bill. One oft-discussed difference is that, unlike familiar forms of property in physical stuff, the subjects of intellectual property rights--the stuff they are rights to--are non-rival. No matter how many individuals read a book, build an engine from a single design, or produce pills from a chemical formula, there is still plenty of book, design, and formula to go around. This is held to raise some serious difficulties for the justification of intellectual property rights. In particular, some hold that the non-rivalrousness of their subject matter allegedly undercuts putative justifications of intellectual property. Because everyone could enjoy books, medicines, designs, and recorded music without precluding anyone else from doing so, there is no reason (other than, perhaps, some instrumental reasons about creating incentives) to exclude anyone from access to these items.