The public trust doctrine is the principle (which began with the Roman Empire and later become common law in the England and the United States) that the air, sea, water and other natural resources is the common property of the public and is to be held in trust for the public.
The responsibility for holding these resources in trust for the public is, according to the doctrine, to lie with the state or nation. This requires states to prevent the loss or destruction of these resources that might result from pollution or from company turning that publicly-owned resource into a commodity for sale or profit.
This principle can be expanded to hold that these resources must be held in trust not only for those living today but those who will be living in the future.
This doctrine stands in contradiction to a number of common government practices, such as allowing companies to redirect water supplies for its own use, granting resource exploitation rights in public lands such as parks and allowing companies to dump waste from their operations into the air or bodies of water.