Traditionally, fishery management has been directed to the objectives of sustainable yield and resource conservation. Since the early 1970s, as a consequence of implementation of the Marine Mammal Protection Act and the Endangered Species Act and their interaction with the Magnuson-Stevens Fishery Conservation and Management Act, the objective of protecting marine mammal populations has been added. The active pursuit of this protection has been hastened by public pressure. Among the ways that marine mammals interact with fisheries are through accidental catch in fishery operations and through direct and indirect competition for prey. The interactions are illustrated in case studies of Steller sea lions and the eastern North Pacific fisheries, dolphins in the tuna fishery of the eastern tropical Pacific, and sea otters and the shellfish fisheries of the California coast. These studies demonstrate both the conflicting legal frameworks and the relative roles of science and societal values in the broadening of fishery management objectives to include an ecological (i.e., multispecies) perspective. The approach now needed (1) recognizes the inherent conflicts in managing marine ecosystems, (2) incorporates uncertainty in a risk averse manner, (3) treats all members of the ecosystem as potentially important, and (4) requires information that is obtainable at an acceptable cost.