As threats to biodiversity mount, the international community is increasingly focusing on conserving diversity. The number of threatened species is an important measure of the immediate need for conservation in an area. Global analyses of the status of threatened species have been carried out for few groups of organisms. Only for mammals, birds, and amphibians has the status of virtually all known species been assessed.

Threatened species are defined using the International Union for Conservation of Nature's (IUCN) classification: endangered (in danger of extinction and unlikely to survive if causal factors continue operating) and vulnerable (likely to move into the endangered category in the near future if causal factors continue operating).

The International Union for Conservation of Nature (IUCN) Red List of Threatened Species is widely recognized as the most comprehensive, objective global approach for evaluating the conservation status of plant and animal species. The IUCN draws on and mobilizes a network of scientists and partner organizations working in almost every country in the world, who collectively hold what is likely the most complete scientific knowledge base on the biology and conservation status of species.

the IUCN Red List covers a comprehensive assessment of the conservation status of the world's 5,488 mammal species, including global summary statistics, individual species accounts/threat category, range map, ecology information, and some other data. Mammal species are found spread across the globe, with the exception of the land mass of Antarctica. Nearly one-quarter of the world's mammal species are known to be globally threatened or extinct, 63 percent are known to not be threatened, and 15 percent have insufficient data to determine their threat status. Habitat loss, affecting over 2,000 mammal species, is the greatest threat globally. The second greatest threat is utilization which is affecting over 900 mammal species, mainly those in Asia.

Direct threats to species are the proximate human activities or processes that have impacted, are impacting, or may impact the status of the taxon being assessed (e.g., unsustainable fishing or logging). Direct threats are synonymous with sources of stress and proximate pressures. Threats can be past (historical, unlikely to return or historical, likely to return), ongoing, and/or likely to occur in the future.
Mammal species, threatened
Reporting the proportion of threatened species on the Red List is complicated by the fact that not all species groups have been fully evaluated, and also by the fact that some species have so little information available that they can only be assessed as Data Deficient (DD). For many of the incompletely evaluated groups, assessment efforts have focused on species that are likely to be threatened; therefore any percentage of threatened species reported for these groups would be heavily biased (i.e., the percentage of threatened species would likely be an overestimate).

Some parts of the world, such as the Andes, Central and West Africa, Angola, parts of South and Southeast Asia, and Melanesia, still have sparse information available of their mammal faunas. In addition, many species' names, especially in the tropics, actually represent complexes of several species that have not yet been resolved. The information on the relative importance of different threatening processes to mammal species is incomplete. IUCN codes all threats that appear to have an important impact, but not their relative importance for each species.

Since IUCN has evaluated extinction risk for less than 5 percent of the world's described species, IUCN cannot provide an overall estimate for how many of the planet's species are threatened. For those groups that have been comprehensively evaluated, the proportion of threatened species can be calculated, but the number of threatened species is often uncertain because it is not known whether Data Deficient species are actually threatened or not.

Due to variations in consistency and methods of collection, data quality is highly variable across countries. Some countries update their information more frequently than others, some have more accurate data on extent of coverage, and many underreport the number or extent of protected areas. Also, because of differences in definitions, reporting practices, and reporting periods, cross-country comparability of threatened species is limited.

In order to ensure global uniformity when describing the habitat in which a taxon (a taxonomic group of any rank) occurs, the threats to a taxon, what conservation actions are in place or are needed, and whether or not the taxon is utilized, a set of standard terms, called Classification Schemes, are being developed, for documenting taxonomy on the IUCN Red List.
Mammal species are mammals excluding whales and porpoises. Threatened species are the number of species classified by the IUCN as endangered, vulnerable, rare, indeterminate, out of danger, or insufficiently known.
Species assessed as Critically Endangered (CR), Endangered (EN) or Vulnerable (VU) are referred to as "threatened" species. The International Union for Conservation of Nature (IUCN) Red List of Threatened Species collects and disseminates information on the global threated species.

Proportion of threatened species is only reported for the more completely evaluated groups (i.e., >90% of species evaluated). Also, the reported percentage of threatened species for each group is presented as a best estimate within a range of possible values bounded by lower and upper estimates:

Lower estimate = % threatened extant species if all Data Deficient species are not threatened, i.e., (CR + EN + VU) / (total assessed - EX)

Best estimate = % threatened extant species if Data Deficient species are equally threatened as data sufficient species, i.e., (CR + EN + VU) / (total assessed - EX - DD)

Upper estimate = % threatened extant species if all Data Deficient species are threatened, i.e., (CR + EN + VU + DD) / (total assessed - EX)

Additional information on ecology and habitat preferences, threats, and conservation action are also collated and assessed as part of Red List process.