Some Members of Congress (among many others) are interested in considering limiting human activity in some areas of the marine environment as one response to mounting evidence of deteriorating conditions and declining populations of living resources. The purposes of proposed additional limits would be to both stem the decline and permit the rehabilitation of these environments and populations. One method of implementing this concept is to designate areas where activity would be limited, often referred to as marine protected areas (MPAs). Translating the MPA approach into a national program, however, requires resolution of many economic, ecological, and social debates. The complexity of creating a program is compounded by controversy over the uses that would be allowed, curtailed, or prohibited in MPAs; the purposes of a system of MPAs; and the location, size, and distribution of MPA units. One possible way to get past some of these complexities is to think of MPA designations as a form of zoning in the ocean. Experiences in using the MPA designation in other countries may be instructive. However, questions have been raised about the effectiveness of administration and enforcement and about changes in living resources at some of these sites. Although the MPA designation has not been used widely in the United States, numerous marine sites have been designated by federal and state governments for some kind of protection. Perhaps the best-known federal sites are units in the National Marine Sanctuary Program. The Bush Administration has supported the MPA concept. It designated the Northwestern Hawaiian Islands Marine National Monument in 2006 as the world's largest MPA and has supported the activities of the National Marine Protected Areas Center in the National Oceanic and Atmospheric Administration. It has continued most of the Clinton Administration initiatives to coordinate protection of marine resources at designated sites, including implementation of Executive Order 13158, issued in May 2000, which endorsed establishing and strengthening a comprehensive system of MPAs. Additional actions by Congress would be needed to create a system that could be characterized as integrated or comprehensive. Recent reports from the Pew Oceans Commission and the U.S. Commission on Ocean Policy endorsed the MPA concept. Some issues that would likely be raised in any congressional discussions include whether new legislation is desired or needed; what the basic characteristics of units in any MPA system should be; how MPAs might be used to resolve use conflicts; and whether adequate funding would be authorized and appropriated to both enforce the protected status and monitor and evaluate the ecological and social impacts of MPAs. Earlier Congresses examined the concepts behind MPAs and experiences with protected areas as they considered appropriations and proposals to reauthorize coastal and marine resource protection laws. However, Congress has not authorized new MPA-related activities, although it addressed fishing access when it reauthorized the Magnuson-Stevens Fishery Conservation and Management Act near the end of the 109th Congress. The 110th Congress is likely to continue considering these topics. This report will be updated as events warrant.