It is important to know or at least be able to estimate the number of people affected by diabetes. Having this knowledge enables us to track and predict the diabetes epidemic so that healthcare can at least attempt to keep pace with the growing numbers (in practice, unfortunately, it rarely can). To have authoritative estimates of the current magnitude of the problem and projections of the likely future burden is of vital importance in continued efforts to make the case for more effective interventions to prevent diabetes and improve the care of those affected by the condition. The authors of this article explore the differences in approach and methodology that are used to make these estimates and projections and explain their impact on differences in detail in the published figures.