In response to the accelerating rate of biodiversity loss, and the far-reaching impacts of this, the governments of 190 countries have pledged to significantly reduce the rate of biodiversity loss by 2010 
. This has led to increasing requirements for indicators that can chart the rate of biodiversity loss 
. In response, the World Conservation Union (IUCN) and its partner organisations developed an indicator - the Red List Index (RLI; 
) - based on the IUCN Red List of Threatened Species™.
The IUCN Red List is widely recognised as the most authoritative and objective system currently available for classifying species in terms of their risk of global extinction 
. It uses quantitative criteria based on population size, rate of decline, and area of distribution to assign species to categories of relative extinction risk 
. These criteria are clear and comprehensive, yet are sufficiently flexible to deal with uncertainty 
. Assessments of individual species using these criteria must be supported by a wealth of documentation, including information on range, occurrence, population, trends, habitat preferences, threats, conservation actions in place and needed 
. The Red List is also becoming increasingly comprehensive, with all species now assessed in several major classes (birds, mammals, amphibians, conifers and cycads) and global assessments underway for all reptiles, marine species in several groups (including sharks and coral-reef fish), several freshwater groups, and selected plant groups (initially, legumes and trees).
The RLI uses information from the IUCN Red List to measure the projected overall extinction risk of sets of species and to track changes in this risk 
. It is based on the proportion of species in each category on the Red List, and changes in this proportion over time resulting from genuine improvement or deterioration in the status of individual species. The RLI was initially designed and tested using data on all bird species from 1988–2004 
, and has since been applied to amphibians 
, with a global mammal RLI in preparation. By 2010, RLI trends will also be available for all conifers and cycads, and for a more representative set of taxa based on a random sample of all vertebrates and selected plant groups. Baseline estimates for reptiles and selected freshwater fish, plant and marine groups will also be available. As well as tracking global trends, the RLI can be disaggregated to show trends for species in different biogeographic realms, political units, ecosystems, habitats, taxonomic groups and for species relevant to different international agreements and treaties.
The RLI has been widely recognised as an important component of the suite of indicators needed to track progress towards the 2010 target 
. Consequently, an indicator on ‘trends in the status of threatened species’ has been moved into the top group of indicators for ‘immediate testing’ by the Convention of Biological Diversity (CBD) Subsidiary Body on Scientific, Technical and Technological Advice (SBSTTA; 
). In addition, RLIs based on the relevant sets of species are currently being considered for adoption by the Ramsar Convention on Wetlands, the Convention on Migratory Species (CMS), the Agreement on the Conservation of Albatrosses and Petrels under the CMS, and for European threatened species through the Streamlining European Biodiversity Indicators-2010 initiative, which is coordinated by the European Environment Agency, the European Centre for Nature Conservation and UNEP-WCMC (the World Conservation Monitoring Centre).
Given this increasing recognition and usage, it is important that the RLI performs well as an indicator, for example, by meeting the criteria for successful indicators described by Gregory et al. 
. Further application of the RLI (and in particular, consideration of its application to non-avian taxa) has, however, revealed some shortcomings in the original formula and approach. We describe these here and recommend revisions that address them to improve the RLI formulation.