The environment has often been thought to consist of resources that are unowned, and hence subject to the well-known tragedy of the commons. But in recent years, property ideas have been increasingly recruited for environmental protection, in a manner that appears to vindicate the view that property rights evolve along with the needs for resource management. Nevertheless, property regimes have some pitfalls for environmental resources: the relevant parties may not be able to come to agreement; property regimes may be weak (...) or ineffective; they may be aimed at purposes inconsistent with environmental protection; property rights definitions may not work well for environmental resources; modern property regimes may promote monoculture rather than diverse environments. This essay describes these problems and asks to what degree they apply to a new effort to use property rights approaches, namely cap-and-trade programs to control greenhouse gases. It concludes that property rights, while imperfect and something of a retreat from a regime of complete liberty, may offer gains for environmental protection. But success will depend on close attention to the accountability and effectiveness of the governmental institutions necessary to support environmental property regimes. (shrink)