A common resource is a good that is nonexcludable but is rival in consumption. An example is the stock of fish in a limited fishing area, like the fisheries off the coast of New England. Traditionally, anyone who had a boat could go out to sea and catch fish—fish in the sea were a nonexcludable good. Yet because the total number of fish is limited, the fish that one person catches are no longer available to be caught by someone else. So fish in the sea are rival in consumption.

Other examples of common resources are clean air and water as well as the diversity of animal and plant species on the planet (biodiversity). In each of these cases the fact that the good, though rival in consumption, is nonexcludable poses a serious problem.