An operational theory of rights, property and property rights is presented with the intent of informing economic analysis and public debate about natural resource and environmental problems. Chapter 2 introduces notions of property rights and property régimes pertinent to natural resource policy. Chapter 3 introduces the protection of entitlements (rights) by property rules, liability rules, and inalienability rules. Chapter 4 examines externalities, including technological, pecuniary, Pareto-irrelevant, Pareto-relevant, infra-marginal, ownership, technical, political, depletable, non-depletable and potentially relevant. Chapter 5 focuses on the problem of intertemporal externalities, and on the public perceptions of environmental uncertainty. Chapter 6 concentrates on resource management regimes, specifically those of the Third World. Common property and the 'tragedy of the commons' are discussed. Chapter 7 challenges the idea that the solution to natural resource degradation is to be found in the establishment of private property regimes in those areas where common property and/or open access are now the rule. Chapter 8 persists with the notion of property rights as policy instruments but turns to the USA to explore different perceptions of rights and duties from an instrumental perspective. Chapter 9 concentrates on the sanctity of private agricultural land. The final chapter is a critical examination of efficiency as an objective truth rule for economic policy evaluation.