This article examines a mode of hybrid governance in which sovereign states and nonstate parties collaborate as equal partners to address complex problems that are beyond the problem-solving capacities of states acting alone. Under conventional state-centric approaches, environmental policy is the exclusive province of territorially delimited sovereign states, subject only to such obligations as states incur through voluntary inter-sovereign agreements. In contrast, “post-sovereign” governance is non-exclusive, non-hierarchical, and post-territorial. These arrangements emerge from recognition of the limitations of top-down domestic regulation and rules of inter-sovereign obligation as means to address such complex environmental problems as ecosystem management. Examples are drawn from the US experience in the Chesapeake Bay region, and the joint US-Canadian Great Lakes ecosystem management effort.