Diabetes mellitus (DM) represents a heterogeneous group of conditions that share certain characteristics with hyperglycemia as a common feature. The first worldwide accepted classification scheme for DM was published in 1979 by the National Diabetes Data Group (NDDG) and classified DM based on the pharmacologic therapy applied into two major groups: Insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM). The terms coined by the NDDG became popular during the 1980s and 1990s, but with time, the misclassification of patients became evident. Since the correct classification of DM allows a more adequate treatment, the new classification proposed by the American Diabetes Association in 1997 was based in the pathogenesis of the disease and comprises four categories: Type 1 DM, Type 2 DM, other types and gestational diabetes. Despite significant advances in diabetes understanding, some gray areas still remain and new studies are necessary to further improve diabetes classification.