Below the stomach resides the pancreas, which resembles the tongue of a large dog. Inside the pancreas are many special cell clusters, which under the microscope are like small islands in the middle of the ocean, and are medically known as islets. The insulin secreted by the beta cells in the pancreatic islets is the only hormone that can lower blood sugar in the body. Diabetes is caused by a problem with the beta cells of the pancreas, which produce insufficient insulin and/or defective insulin action. Different types of diabetes have different ways of islet beta cell damage. Type 1 diabetes develops due to an autoimmune response that results in massive destruction of pancreatic islet beta cells, which cannot produce sufficient insulin to lower blood glucose. Type 2 diabetes usually has a genetic background, combined with environmental factors (e.g., obesity, mental stimulation, infection, trauma, etc.), resulting in reduced insulin action (i.e., insulin resistance). At the same time, pancreatic β-cells compensatingly work twice as hard to maintain blood glucose level, and after a long time, pancreatic β-cells fail to function, resulting in insufficient insulin secretion. Type 2 diabetes arises when blood glucose cannot be controlled at physiological levels due to insulin resistance and/or insufficient insulin secretion. The main problem of gestational diabetes is that the secretion of various insulin-fighting hormones in the body increases after pregnancy, and the pregnant woman’s own insulin secretion cannot maintain blood glucose at physiological levels, thus leading to diabetes. Other specific types of diabetes include defective pancreatic beta-cell function, defective insulin function, extra-pancreatic diseases, endocrine diseases, rare immune-mediated diabetes, and diabetes accompanied by some genetic diseases, certain infectious diseases, and diabetes caused by drugs and chemicals.