Diabetes mellitus is diagnosed and characterized by chronic hyperglycemia. The effects of diabetes mellitus include long-term damage, dysfunction, and failure of various organs, especially the eyes, kidneys, heart, and blood vessels. Often diabetic symptoms are not severe or may even be absent. Hyperglycemia sufficient to cause pathologic functional changes may quite often be present for a long time before the diagnosis is made. Because diabetes mellitus is a chronic progressive disease, early diagnosis of diabetes is important to prevent chronic diabetic complications, especially in high risk subjects. In most countries, screening methods for the early diagnosis of diabetes have not yet been agreed. The fasting plasma glucose is simple, quick, acceptable to patients, and of low cost, but can miss those with isolated post-challenge hyperglycemia. The oGTT is difficult to perform, impractical for large numbers, and expensive, but is the only way to identify post-load hyperglycemia. Attention is focused on those at high risk of developing diabetes.