Before 1994, the more subjective threatened species categories used in IUCN Red Data Books and Red Lists had been in place, with some modification, for almost 30 years. In 1989, development of a more objective approach began, and in 1994 the first IUCN Red List Categories and Criteria (version 2.3) were adopted by IUCN Council.
A review process began in 1996 to address improvements to the criteria identified by Assessors. This resulted in a revised version of the criteria (version 3.1), which was adopted by IUCN Council in February 2000. The IUCN Red List Categories and Criteria. Version 3.1 provides a clearer, more open and easy-to-use system.
The conversion of all assessments on the IUCN Red List that use the 1994 system (version 2.3) to the current system (version 3.1) will take time. Meanwhile, the current IUCN Red List includes assessments using both the 1994 system and the 2001 system. To avoid confusion, all assessments using the 2001 Red List Categories and Criteria are indicated by a 'ver 3.1 (2001)' hyperlink, and all assessments using the 1994 Red List Categories and Criteria are indicated by a 'ver 2.3 (1994)' hyperlink.