ECONOMISTS HAVE STUDIED NATURAL RESOURCES from the earliest days of the profession and for good reason. Resources are seen as the basis for national prosperity, power, and wealth. For example, the ability to harness energy resources in new ways is recognized as perhaps the major factor underlying the industrial revolution. Even more fundamental, food supplies depend on forests, fisheries, and agricultural land. Yet only relatively recently have broad theories been developed specific to natural resources and energy economics. Previously, examination of these fields relied upon the general economic theories used to analyze other commodities. Now, however, economists recognize that certain special characteristics of natural resources require theories that account explicitly for these characteristics. Only in the last generation have agricultural land, forests, and fisheries been perceived and described as renewable resources. Of course, this does not imply unconditional renewal. Such resources are self-renewing at a limited rate, which may itself depend on the size of the stock in existence at any given time and on the extent and nature of human intervention into the stock dynamics. Regardless, renewable resources is an apt and useful term. In contrast, minerals and many energy commodities now are seen as depletable or nonrenewable resources-resources for which only a limited, concentrated stock exists for allocation over all time. For these resources, a central issue involves when they should be extracted and used. Using a given portion of a stock at one time precludes the option of using that portion at another time. Even more recently have the environmental resources-air, water, open space-also been seen as renewable or, in some cases, depletable resources. The image of environmental resources, fisheries, and wild animal stocks as common property resources owned by everyone and hence by no one has played an enormous role in economic reasoning about these resources in recent decades. Furthermore, economists systematically have in-