The etiology and pathogenesis of diabetes mellitus are extremely complex and have not yet been fully elucidated. The etiology varies from type to type and even within the same type there is variability. Insulin is synthesized and secreted by pancreatic β-cells and reaches the target cells in various tissues of the body through blood circulation, binds to specific receptors and triggers the metabolic effects of substances in the cells. The causes are not the same for different types. Type 1 diabetes: abnormal pancreatic β-cell function or apoptosis caused by genetic factors or autoimmune defects, and insufficient insulin secretion. In addition, environmental factors such as viral infection and stimulation by harmful substances can also cause the destruction of beta cells. Type 2 diabetes: Insulin resistance and functional defects of pancreatic β-cells caused by acquired factors, which are both congenital and acquired, such as obesity and metabolic syndrome. Gestational diabetes: This category is generally due to changes in basal metabolism and hormone levels during pregnancy, resulting in disorders of glucose metabolism, which may first appear during pregnancy or may be triggered after pregnancy by previous high-risk factors. It may be related to the infection of glucose metabolism by chorionic gonadotropin (HCG) and estrogen. Other types: With the advancement of medical technology, some special types of diabetes have been discovered clinically, with characteristics of both type 1 and type 2 diabetes, such as autoimmune diabetes, and diabetes caused by some special etiologies such as abnormal secretion of proinsulin, abnormal function of pancreatic alpha cells, abnormal intestinal flora, etc. However, the specific pathogenesis are under research and are inconclusive. Therefore, diabetes mellitus is a large group of diseases characterized by elevated blood glucose. The etiology and pathogenesis of diabetes mellitus are extremely complex, and the treatment is highly individual, so patients need to communicate with their doctors repeatedly and actively cooperate to find a suitable treatment plan.