An extensive review of the categories and criteria used to list species on the IUCN Red List, called for in 1996, has been completed by the Species Survival Commission (SSC). The revised Red List Categories and Criteria were published in 2001, with a second edition released in 2012, and are available in IUCN's three official languages:  English; Français; Español

The review, involving broad consultation with users and organizations from around the world, has produced a clearer, more open, and easy-to-use system. With particular attention paid to marine species, harvested species, and population fluctuations, the review has refined the effectiveness of the Red List Categories and Criteria as indicators of extinction risk.

The IUCN Red List System was first conceived in 1963 and set a global standard for species listing and conservation assessment efforts. For more than 30 years SSC has been evaluating the conservation status of species and subspecies on a global scale - highlighting those threatened with extinction and promoting their conservation.

Over time, IUCN recognized that a more objective and scientific system for determining threat status, as well as a more accurate system for use at the national and regional level were needed. The IUCN Red List Categories evolved over a four-year period through extensive consultation and testing involving more than 800 SSC members, and the wider scientific community. The more precise and quantitative Red List Categories were adopted by IUCN in 1994.

Then in 1996, IUCN members called for a further review to ensure that the criteria were applicable to a wide range of organisms, especially long-lived species, and species under intensive management. In addition, SSC was asked to ensure the highest standards of documentation (information supplied to justify a listing), information management, and scientific credibility.

The revised Categories were adopted by IUCN Council in February 2000 and, following further refinement, have been published. They will help place SSC and IUCN at the forefront of biodiversity analyses that contribute to scientific discovery and to political policies related to conservation at local, national and regional levels.

New areas of conservation biology research have been spawned by the review process, and many papers have already appeared in the scientific literature about the use of the IUCN Red List Categories and Criteria. SSC will leave this system unchanged for a period long enough to allow genuine changes in conservation status to be monitored. Stability in the categorization system is essential if the IUCN Red List is to be used as a reliable indicator of trends in biological diversity.

The Authority Files used for the documentation standards as described in Annex 3 of the Categories and Criteria. Guidelines for Using the IUCN Red List Categories and Criteria are also available and address many of the issues raised in the process of reviewing the 1994 Categories and Criteria. These guidelines explain how the IUCN Red List Categories and Criteria should be applied to determine whether a taxon belongs in a category of threat, and give examples from different taxonomic groups to illustrate the application of the criteria. They also provide detailed explanations of the definitions of the many terms used in the criteria. The guidelines should be used in conjunction with the official IUCN Red List Categories and Criteria booklet (IUCN 2001). Note that these guidelines differ from the Guidelines for Application of IUCN Red List Criteria at Regional Levels, which apply to regional and national assessments.