Diabetes mellitus is an endocrine metabolic disorder with the characteristic feature of hyperglycemia i.eincreased glucose levels. It usually occurs due to the disturbances in carbohydrate metabolism. Glucose isan important aldohexose sugar whose level gets raised during diabetes. Normally in the body glucose isacted upon by the hormone Insulin which is secreted by the beta islet cells of the pancreas. Howeverduring diabetes either the level of insulin decreases or the cells become insensitive to the action of insulin.Diabetes mellitus is associated with a number of complications affecting eyes, kidney, heart, nerves etc. Alot of research is done or is still is going on, but diabetes mellitus still is the leading cause of deathworldwide. The number of diabetic cases in developing countries is increasing day by day and no hugeeffort has been put into action to stop this sweet but deadly disease. Diabetes mellitus can be controlled bydiet, lifestyle, exercise, oral hypoglycemic agents or insulin (pharmacological). Diabetes mellitusselectively damages cells(endothelial cells and mesangial cells) because in these cells the rate of glucosetransport does not decline rapidly as a result of hyperglycemia, leading to high glucose inside the cell.Diabetes mellitus not only affects small blood vessels but also large blood vessels, so the complications ofdiabetes are divided into two classes i.e microvascular(small blood vessels) and macrovascular (largeblood vessels).These complications affect and cause severe damage not only to the organs but to thenerves as well. So such medications should be developed that at least should minimize these adversedamages. At the moment insulin is widely used in diabetes in combination with other medicines. Not onlydrugs but products obtained from natural sources which could act as a protective agents should be lookedupon to reverse and decrease such adverse damages. In this review main focus has been given on insulinmolecule(synthesis, secretion, action,etc) and its role in diabetes mellitus.