The 2023 UNAIDS HIV estimates, developed using methods guided by the UNAIDS Reference Group on Estimates, Modeling, and Projections, cover 174 countries, accounting for 99% of the global population. These estimates are based on updated methods and epidemiological analyses. The supplement summarizes the methods of tools in the Spectrum software package, which is a core tool for HIV modeling. It introduces new indicators for district-level HIV treatment targets and describes updated methods for estimating new infections among key populations, including sex workers, men who have sex with men, people who inject drugs, transgender people, and clients of female sex workers. New epidemiological analyses are also presented, covering topics such as breastfeeding duration among mothers living with HIV, mortality rates of those living with HIV on antiretroviral treatment, and trends in antenatal HIV testing and prevalence. The 2023 HIV estimates show that as of the end of 2022, 39.0 million people were living with HIV, with 53% being women and girls. Sixty-five percent of people living with HIV reside in sub-Saharan Africa, but only half of new HIV infections were in this region. There were 1.3 million new HIV infections in 2022, with 630,000 AIDS-related deaths. The UNAIDS estimates also track progress toward the 95-95-95 targets, which aim to ensure 95% of people living with HIV know their status, 95% of those who know their status are on treatment, and 95% of those on treatment achieve viral suppression. In 2022, the global achievement was 86%-89%-93%, a significant improvement from 2015. The Spectrum software package includes the Demographic Projection Model (DemProj) and the AIDS Impact Model (AIM), which are used to model national populations and assess the impacts of HIV. The Naomi model is used to estimate PLHIV, treatment coverage, and new infections at the district level in sub-Saharan Africa. The supplement also discusses new methods for estimating new infections among key populations and highlights the importance of accurate data and models for effective HIV response. The data and methods used to derive the estimates continue to evolve in response to new information and changing epidemiology. However, there is wide variability in the quality of underlying HIV data, which affects the accuracy of estimates. The supplement emphasizes the need for improved data and analysis to address inequalities and ensure effective HIV response.The 2023 UNAIDS HIV estimates, developed using methods guided by the UNAIDS Reference Group on Estimates, Modeling, and Projections, cover 174 countries, accounting for 99% of the global population. These estimates are based on updated methods and epidemiological analyses. The supplement summarizes the methods of tools in the Spectrum software package, which is a core tool for HIV modeling. It introduces new indicators for district-level HIV treatment targets and describes updated methods for estimating new infections among key populations, including sex workers, men who have sex with men, people who inject drugs, transgender people, and clients of female sex workers. New epidemiological analyses are also presented, covering topics such as breastfeeding duration among mothers living with HIV, mortality rates of those living with HIV on antiretroviral treatment, and trends in antenatal HIV testing and prevalence. The 2023 HIV estimates show that as of the end of 2022, 39.0 million people were living with HIV, with 53% being women and girls. Sixty-five percent of people living with HIV reside in sub-Saharan Africa, but only half of new HIV infections were in this region. There were 1.3 million new HIV infections in 2022, with 630,000 AIDS-related deaths. The UNAIDS estimates also track progress toward the 95-95-95 targets, which aim to ensure 95% of people living with HIV know their status, 95% of those who know their status are on treatment, and 95% of those on treatment achieve viral suppression. In 2022, the global achievement was 86%-89%-93%, a significant improvement from 2015. The Spectrum software package includes the Demographic Projection Model (DemProj) and the AIDS Impact Model (AIM), which are used to model national populations and assess the impacts of HIV. The Naomi model is used to estimate PLHIV, treatment coverage, and new infections at the district level in sub-Saharan Africa. The supplement also discusses new methods for estimating new infections among key populations and highlights the importance of accurate data and models for effective HIV response. The data and methods used to derive the estimates continue to evolve in response to new information and changing epidemiology. However, there is wide variability in the quality of underlying HIV data, which affects the accuracy of estimates. The supplement emphasizes the need for improved data and analysis to address inequalities and ensure effective HIV response.