With the smaller number of marine protected areas (MPAs) compared with terrestrial protected areas, there is less experience and understanding of applying the categories to MPAs. Application of the categories to MPAs has often been inaccurate and inconsistent. These supplementary marine guidelines are thus aimed at ensuring that the IUCN categories can be effectively applied to all types of MPAs as well as to any marine components of adjoining terrestrial protected areas, provided a site meets the IUCN definition of a protected area. Inconsistencies in the application of, and reporting on, the categories reduce the efficacy and use of the system as a global classification scheme. These supplementary guidelines should increase the accuracy and consistency of both assignment and reporting.