Abstract: Should property rights be protected absolutely -- by property rules -- or instead by the requirement that infringing parties pay for harm done--that is, by liability rules? In this article, we present a systematic economic analysis of this fundamental question. Our primary object is to explain why liability rules are often employed to protect individuals against harmful externalities (such as pollution and automobile accidents), whereas property rules are generally relied upon to protect individuals from having their possessions taken from them, thereby ensuring a basic incident of ownership. In the course of our analysis, we suggest that a variety of commonly held beliefs about property and liability rules are in error, and we also derive results bearing on legal policy. Notably, we show that, for controlling some important externalities, liability rules (and pollution taxes) are superior to property rules (including many forms of regulation) even when damages must be set using only limited information about harm.