In the 2001 IUCN Red List Categories and Criteria (version 3.1), seven threat categories are defined as follows:
| Extinct (EX) | A taxon is Extinct when there is no reasonable doubt that the last individual has died. |
|---|---|
| Extinct in the wild (EW) | A taxon is Extinct in the wild when it is known of survive only in cultivation, in captivity or as a naturalized population well out side the past range. |
| Critically Endangered (CR) | A taxon is Critically Endangered when it is facing an extremely high risk of extinction in the wild in the immediate future, as detailed by any of the criteria A to E (see table below). |
| Endangered (EN) | A taxon is Endangered when it is not Critically Endangered but is facing a very high risk of extinction in the wild in the near future, as defined by any of the criteria A to E. |
| Vulnerable (VU) | A taxon is Vulnerable when it is not Critically Endangered or Endangered but is facing a high risk of extinction in the wild in the medium term future, as defined by any of the criteria A to E. |
| Near Threatened (NT) | A taxon is Near Threatened when it has been evaluated against the criteria but does not qualify forCritically Endangered, Endangered orVulnerable now, but is close to qualifying for or is likely to qualify for a threatened category in the near future. |
| Least Concern (LC) | A taxon is Least Concern when it has been evaluated against the criteria and does not qualify for Critically Endangered, Endangered, Vulnerable orNear Threatened. |
The qualifying thresholds within the criteria A to E differ among three of thethreat categories. They are summarized as follows:
| Criterion | Critically Endangered | Endangered | Vulnerable | |
|---|---|---|---|---|
| A | Rapid decline of population | >80% over 10 yrs or 3 generations in past or future | >50% over 10 yrs or 3 generations in past or future | >30% over 10 yrs or 3 generations in past or future |
| B | Small distribution: Fragmented, declining or fluctuating | extent of occurrence <100 km2 or area of occupancy <10 km2 | extent of occurrence <5000 km2 or area of occupancy <500 km2 | extent of occurrence <20000 km2 or area of occupancy <2000 km2 |
| C | Small population and declining | <250 mature individuals or population decline 25% in 3 yrs or 1 generation. | <2500 mature individuals or population decline20% in 5 yrs or 2 generations. | <10000 mature individuals or population decline 10% over 10 yrs or 3 generations. |
| D | Very small population | <50 mature individuals | <250 mature individuals. | <1000 mature individuals or area of occupancy <20 km2, or locations <5. |
| E | Quantitative analysis (Probability of extinction) | >50% in 10 yrs or 3 generations. | >20% in 20 yrs or 5 generations. | >10% in 100 yrs. |