Every year, UNAIDS supports countries to generate national estimates and projections of trends and impacts of the HIV epidemic. Estimates are created by fitting mathematical models to locally available HIV surveillance data and other programmatic data about the national HIV response. This seminar will provide an introduction to the methods and models that underpin UNAIDS global HIV estimates implemented in the Spectrum software, and the Thembisa model, a bespoke model developed for the HIV epidemic and intervention impacts in South Africa.