Natural resources are generally referredto as ‘‘depleteable’’ and‘‘renewable.’’ The term ‘‘renewable’’ typically refers to ‘‘biological’’ resources, such as fish and forests. Other renewable resources, including hydro, solar, tidal, wave and wind power are not explicitly addressed by existing economic theories. Identification of the physical and economic characteristics of natural resources enables the real cost of natural resource use to be identified, providing the basis of a single unifying theory of all resources based on a user cost function. Two key considerations emerge: firstly, in relation to resource depletion, it is necessary to explore the role of rate of consumption relative to rate of supply as a quantitative aspect of resources; secondly, in relation to pollution, it is necessary to investigate the role of rate of change of entropy as a resource is utilisedas a qualitative aspect of resources. Once the true costs of resource use have been established, natural resources can be classified in a systematic way. Clear differences between different types of natural resources emerge, leading to new terms being proposed to describe different resource types.