Marine Protected Areas (MPAs) are spatially delimited areas of the marine environment that are managed at least in part, for conservation of biodiversity. The number of MPAs declared worldwide is increasing exponentially Because MPAs can be declared for a variety of reasons, the specific goals of each MPA need to be specified to allow management agencies to assess the success of the MPA, and to guide monitoring and research activities within. Because of the complexity of processes within marine ecosystems, ecological changes associated with the declaration of MPAs vary greatly from one region to another and are difficult to predict accurately. Important factors that affect the way plants and animals respond to MPAs include distribution of habitat types, level of connectivity to nearby fished habitats, wave exposure, depth distribution, prior level of resource extraction, regulations, and level of compliance to regulations. The value of MPAs primarily relates to biodiversity conservation, fisheries, and as research and management tools, but they can also generate recreational, aesthetic and educational benefits. Conservation benefits are evident through increased habitat heterogeneity at the seascape level, increased abundance of threatened species and habitats, and maintenance of a full range of genotypes. Fisheries can benefit through spillover, increased dispersal of egg and larval propagules, and as insurance against stock collapse. Scientific benefits primarily relate to the use of MPAs as reference areas to assess the scale of human impacts on the environment, and as locations for the collection of data that are unobtainable in fished systems. Nevertheless, MPAs can also involve costs to human society through displaced fishing effort, short-term reductions in catches, false security, and through undesirable interactions within the biota. MPAs do not represent the universal panacea for all threats affecting marine ecosystems, but can be regarded as arguably the most important tool in the marine manager's toolbox.