The paper gives a new characterization of solutions of the decentralized supervisory control problem and presents a special class of "fully decentralized" solutions, so called because the main idea is to avoid communication not only among subsupervisors during run-time but also among agents designing the subsupervisors. The solutions are constructed from optimal centralized ones using a worst-case philosophy, Their basic properties and comparison with the partially observable supervisory control problem are given. if it is known a priori that the fully decentralized solution exists, then the supervisor can be computed partly on-line. A sufficiency test of the existence of such a solution is provided.