The aim of this chapter is to focus on the glucose tolerance test. The glucose tolerance test (GTT) is a test to determine how quickly glucose is cleared from the blood after a load of glucose. It is used to detect diabetes, insulin resistance and sometimes reactive hypoglycemia and acromegaly, or rarer disorders of carbohydrate metabolism. This chapter covers the preparation, procedure, results, oral glucose challenge test (OGCT), renal glycosuria, and alimentary glycosuria or alimentary diabetes or digestive glycosuria.