Understanding the prevalence of HIV infection and its trends over time is essential for assessing the size of the viral reservoir in the population, forecasting and planning the resources needed for patient care, and, more broadly, evaluating the socioeconomic impact of this infection. Since prevalence measurements based on serological screening in a representative sample of the population can lead to highly biased estimates due to unequal participation of different exposure groups, it is necessary to use other approaches, notably the direct method and the back-calculation method, the principles of which are outlined in the article. The first figures obtained using these two methods date back to 1989. Prevalence was then estimated at approximately 150,000 (85,000–199,000). Prevalence in 1992, obtained through similar calculations, was estimated to be between 77,000 and 145,000. This article summarizes a study by Coordinated Action 23 of the French National Agency for AIDS Research (ANRS), "Epidemic Dynamics," which updates these estimates for the late 1990s and for which a detailed report is available. Since seroprevalence surveys were discontinued after 1997, the direct method could not be applied beyond that year. The back-calculation method allowed for estimates up to the year 2000. (inspired by R.A.)