This article seeks to provide in property legal theory an alternative to law-and-economics theory, the dominant mode of theorizing about property in contemporary legal scholarship. I call this alternative the social obligation theory. I argue that American property law, both on the private and public sides, includes a social-obligation norm but that this norm has never been explicitly recognized as such nor systemically developed. I argue that a proper understanding of the social obligation explains a remarkably wide array of existing legal doctrine in American property law, ranging from the power of eminent domain to the modern public trust doctrine. In some cases social obligation reaches the same result as law-and-economics, but in other...