The Reference Group was formed in 1997 with a secretariat established in 1999 to formalize and streamline its operation. The Group advises on the development and refinement of the estimation and projection software package (EPP and Spectrum) used by UNAIDS to estimate the national and global burden of HIV. The Reference Group seeks to balance practicality and scientific rigour to make recommendations based on the best-available evidence. To do this it organises international meetings of experts to advise on topics of contemporary relevance which will impact upon HIV epidemiology and estimation of HIV incidence, prevalence, AIDS deaths, need for treatment and other epidemiological indicators. Rather than having a formal membership, meeting participants are invited to attend meetings where their expertise is relevant.