In this article, the author reviews current approaches and methods for measuring the scope of the human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) epidemic and their strengths and weaknesses. In recent years, various public health agencies have revised statistical estimates of the scope of the HIV/AIDS pandemic. The author considers the reasons underlying these revisions. New sources of data for estimating HIV prevalence have become available, such as nationally representative probability-based surveys. New technologies such as biomarkers that indicate when persons became infected are now used to determine HIV incidence rates. The author summarizes the main sources of errors and problems with these and other approaches and discusses opportunities for improving their reliability. Changing methods and data sources present new challenges, because incidence and prevalence estimates produced at different points in time are not directly comparable with each other, which complicates assessment of time trends. The methodological changes help explain the changes in global statistics. As methods and data sources continue to improve, the development of statistical tools for better assessing the extent to which changes in HIV/AIDS statistics can be attributed to changes in methodology versus real changes in the underlying epidemic is an important challenge.