The pathogenesis of diabetes mellitus involves complex mechanisms centered around insulin deficiency, resistance, or both. In type 1 diabetes mellitus (T1DM), an autoimmune destruction of pancreatic beta cells leads to insulin deficiency, resulting in hyperglycemia. Conversely, type 2 diabetes mellitus (T2DM) is characterized by insulin resistance in peripheral tissues, combined with impaired insulin secretion from pancreatic beta cells. Both genetic predisposition and environmental factors such as obesity and a sedentary lifestyle contribute to the development of T2DM. Additionally, gestational diabetes mellitus (GDM) occurs during pregnancy due to hormonal changes that increase insulin resistance. The pathophysiology of diabetes mellitus underscores the importance of insulin regulation and glucose metabolism in maintaining normal physiological function and highlights the diverse etiological factors involved in its development.