Diabetes mellitus is one of the most important public health challenges of the twenty-first century. Until the past decade, it has been seriously underrated as a global health threat. Major gaps exist in efforts to comprehend the burden nationally and globally, especially in developing nations, due to a lack of accurate data for monitoring and surveillance. Early attempts to obtain accurate data, discussed in this article, seem to have been cast aside so, at present, these needs remain unmet. Existing international efforts to assemble information fall far short of requirements. Current estimates are imprecise, only providing a rough picture, and probably underestimate the disease burden. The methodologies that are currently used, and that are discussed in this Perspectives article, are inadequate for providing a complete and accurate assessment of the prevalence of diabetes mellitus. International consensus on uniform standards and criteria for reporting national data on diabetes mellitus prevalence as well as for common complications of diabetes mellitus and mortality need to be developed.