The public trust doctrine is a legal doctrine dealing with the protection of certain uses and resources for public purposes, regardless of ownership. These uses or resources must be made available to the public, regardless of whether they are under public or private control. That is, they are held in trust for the public.

The modern public trust doctrine can be traced back at least as far as Roman civil law. It was further developed in English law as "things common to all." This public trust was applied mostly to navigation: the Crown possessed the ocean, the rivers, and the lands underlying these bodies of water (referred to as jus publicum). These were to be controlled in order to guarantee the public benefit of free navigation.

In the United States, the public trust doctrine is rooted in Illinois Central Railroad Co. v. Illinois (1892). In this case...