Species are classified in nine groups, set through criteria such as rate of decline, population size, area of geographic distribution, and degree of population and distribution fragmentation.

• Extinct (EX) - No individuals remaining.
• Extinct in the Wild (EW) - Known only to survive in captivity, or as a naturalized population outside its historic range.
• Critically Endangered (CR) - Extremely high risk of extinction in the wild.
• Endangered (EN) - High risk of extinction in the wild.
• Vulnerable (VU) - High risk of endangerment in the wild.
• Near Threatened (NT) - Likely to become endangered in the near future.
• Least Concern (LC) - Lowest risk. Does not qualify for a more at risk category. Widespread and abundant taxa are included in this category.
• Data Deficient (DD) - Not enough data to make an assessment of its risk of extinction.
• Not Evaluated (NE) - Has not yet been evaluated against the criteria.^[5]

When discussing the IUCN Red List, the official term "threatened" is a grouping of three categories: Critically Endangered, Endangered, and Vulnerable.