Diabetes mellitus now occurs in at least 100 million people worldwide, and it is estimated that by 2010 this will have more than doubled to 230 million. The impact of this increase will be far reaching through the micro- and macrovascular complications of diabetes, which lead to blindness, amputation, renal failure, and cardiovascular disease. Epidemiology is but one branch of medical science involved in the study of diabetes and is directed mainly at understanding the changing disease distribution, the causes and risk factors of diabetes, and the likely effect of interventions to prevent diabetes or its complications. Classification and diagnostic criteria for diabetes are integral to diabetes epidemiology because changes in these can dramatically alter prevalence estimates; at the same time, however, epidemiology can inform the debate on revised classification systems for diabetes. This review examines some of the key issues in diabetes epidemiology today: the National Diabetes Data Group and World Health Organization revised diabetes classifications based on etiology, the anticipated epidemic of diabetes, and factors contributing to increases in both Type 1 and Type 2 diabetes, complications of diabetes, and issues involved with diabetes screening. In the absence of effective interventions for both Type 1 diabetes and Type 2 diabetes, the disease appears set to escalate world-wide with the main impact being seen in developing nations. Thus, finding ways to prevent or delay the onset of diabetes and finding effective treatment to prevent its microvascular and macrovascular complications are essential components of future public health strategies for all nations.