All type 1 diabetes mellitus, is the main indication for insulin. Regardless of the presence of acute and chronic complications, lifelong insulin replacement therapy is required and cannot be terminated abruptly. If type 2 diabetes is not well controlled by diet and oral hypoglycemic therapy, and if physical exercise and diet therapy are not effective, insulin therapy can be added directly. The addition of insulin can be considered for the first-onset type 2 diabetes, especially the wasting type. Acute metabolic disorders in diabetes, including ketoacidosis, non-ketotic hyperosmolar coma and lactic acidosis, can be used. Diabetic patients with severe infection, trauma, undergoing surgical treatment, acute myocardial infarction and cerebrovascular accident are indications for insulin therapy, which can be discontinued and adjusted to the original treatment regimen after the stressful state. Diabetic patients with chronic complications, such as proliferative retinopathy, severe neuropathy, diabetic nephropathy, cardiac lesions, severe skin lesions and liver cirrhosis and hepatitis, should be treated with insulin. Patients with diabetes, including those with gestational diabetes, are treated with insulin during pregnancy and delivery to ensure fetal development. Insulin therapy is appropriate for type 2 diabetic patients with wasting diseases such as tuberculosis and tumors. Malnutrition-related diabetes mellitus, various secondary diabetes mellitus such as pancreatic-derived diabetes mellitus, pituitary GH tumor, Cushing’s syndrome, steroidal diabetes mellitus and insulin gene mutation diabetes mellitus should be treated with insulin. Clinically similar to diabetes but positive for islet cell antibodies or anti-glutamic acid decarboxylase antibodies in the blood, such as late-onset autoimmune diabetes, is treated with insulin.