The 2023 UNAIDS HIV estimates, supported by the Joint United Nations Programme on HIV/AIDS (UNAIDS), cover 174 countries, accounting for 99% of the global population. The methods used to derive these estimates are developed under the guidance of the UNAIDS Reference Group on Estimates, Modeling, and Projections. This supplement documents updates to these methods and epidemiological analyses that inform parameters and assumptions. Key highlights from the 2023 estimates include: - 39.0 million people living with HIV as of the end of 2022, a slight increase due to prolonged life expectancy and ongoing new infections. - 53% of those living with HIV are women and girls. - 65% of people living with HIV reside in sub-Saharan Africa, but only half of new infections occur there. - 1.3 million new HIV infections in 2022, with a decline in the pace insufficient to meet UNAIDS targets. - 630,000 AIDS-related deaths in 2022, despite the availability of antiretroviral treatment (ART). - Progress toward the 95-95-95 targets (95% knowing status, 95% on treatment, 95% on treatment achieving viral suppression) was 86%-89%-93%. The Spectrum software package, a core tool for HIV estimation, includes the Demographic Projection Model (DemProj) and the AIDS Impact Model (AIM). New indicators for district-level HIV treatment targets and updated methods for estimating new infections among key populations (sex workers, men who have sex with men, people who inject drugs, transgender people, and clients of female sex workers) are introduced. Epidemiological analyses, such as breastfeeding duration among mothers living with HIV and mortality rates among those on ART, are integrated into the models to improve accuracy. However, the quality and confidence in estimates vary significantly across locations and populations, with many regions lacking high-quality, timely data. The conclusion emphasizes the need for renewed commitments to improve data and use existing data to address inequalities and ensure equitable monitoring and response to HIV.The 2023 UNAIDS HIV estimates, supported by the Joint United Nations Programme on HIV/AIDS (UNAIDS), cover 174 countries, accounting for 99% of the global population. The methods used to derive these estimates are developed under the guidance of the UNAIDS Reference Group on Estimates, Modeling, and Projections. This supplement documents updates to these methods and epidemiological analyses that inform parameters and assumptions. Key highlights from the 2023 estimates include: - 39.0 million people living with HIV as of the end of 2022, a slight increase due to prolonged life expectancy and ongoing new infections. - 53% of those living with HIV are women and girls. - 65% of people living with HIV reside in sub-Saharan Africa, but only half of new infections occur there. - 1.3 million new HIV infections in 2022, with a decline in the pace insufficient to meet UNAIDS targets. - 630,000 AIDS-related deaths in 2022, despite the availability of antiretroviral treatment (ART). - Progress toward the 95-95-95 targets (95% knowing status, 95% on treatment, 95% on treatment achieving viral suppression) was 86%-89%-93%. The Spectrum software package, a core tool for HIV estimation, includes the Demographic Projection Model (DemProj) and the AIDS Impact Model (AIM). New indicators for district-level HIV treatment targets and updated methods for estimating new infections among key populations (sex workers, men who have sex with men, people who inject drugs, transgender people, and clients of female sex workers) are introduced. Epidemiological analyses, such as breastfeeding duration among mothers living with HIV and mortality rates among those on ART, are integrated into the models to improve accuracy. However, the quality and confidence in estimates vary significantly across locations and populations, with many regions lacking high-quality, timely data. The conclusion emphasizes the need for renewed commitments to improve data and use existing data to address inequalities and ensure equitable monitoring and response to HIV.