Diabetes mellitus is a chronic metabolic disorder. They are of two types i.e. insulin-dependent diabetes mellitus (IDDM) and non-insulin dependent diabetes mellitus (NIDDM). Prolonged hyperglycemia leads to premature atherosclerosis, retinopathy, nephropathy, gangrene of limbs, etc. Insulin is discovered by Banting and Best in 1921. They consist of two polypeptide chain, i.e. chain A and B. Insulin in β cell is initially a single polypeptide chain, i.e. preproinsulin. Glucose transporters are proteins present on different tissue. Glucose enters pancreatic β cells with help of glucose transporters. Oral antidiabetic agents are classified into sulfonylureas, biguanides, meglitinides, thiazolidinediones, and alpha glucosidase inhibitors. All these classifications are discussed briefly in this chapter. Management of diabetes mellitus is also covered in this chapter. At the end of this chapter there are 30 questions along with their crisp and to the point answers have been also given, which will be of benefit for answering the short answers and even a viva voce question.