Diabetes mellitus (DM) is a chronic metabolic disorder characterized by persistent hyperglycaemia resulting from impaired insulin secretion, insulin resistance, or a combination of both. It represents a major global public health challenge because of its increasing prevalence and its association with substantial morbidity, mortality, and healthcare burden. Diabetes mellitus comprises several distinct forms, including type 1 diabetes mellitus (T1DM), type 2 diabetes mellitus (T2DM), gestational diabetes mellitus (GDM), and specific forms such as latent autoimmune diabetes in adults (LADA), maturity-onset diabetes of the young (MODY), and neonatal diabetes mellitus. Prediabetes represents an intermediate state of dysglycaemia and is an important risk factor for the development of T2DM. The pathogenesis of diabetes is multifactorial and involves genetic susceptibility, autoimmune mechanisms, insulin resistance, pancreatic β-cell dysfunction, obesity, environmental influences, and metabolic abnormalities. Persistent hyperglycaemia can result in acute complications, including hypoglycaemia, diabetic ketoacidosis, and hyperosmolar hyperglycaemic state, as well as chronic microvascular and macrovascular complications affecting the kidneys, eyes, nervous system, cardiovascular system, and peripheral vasculature. Early diagnosis using fasting plasma glucose, oral glucose tolerance testing, glycated haemoglobin, and appropriate clinical assessment is essential for timely intervention. Management involves individualized lifestyle modification, nutritional therapy, physical activity, glucose-lowering pharmacotherapy, and insulin therapy when indicated. Recent advances have expanded the therapeutic landscape through incretin-based therapies, dual GIP/GLP-1 receptor agonists, SGLT2 inhibitors with cardiorenal benefits, novel insulin formulations, immunotherapy for T1DM, stem-cell-derived β-cell replacement, continuous glucose monitoring, automated insulin delivery, and digital health technologies. Current approaches increasingly emphasize patient-centred and personalized treatment, with therapeutic selection guided not only by glycaemic control but also by cardiovascular, renal, metabolic, and quality-of-life considerations. Continued research into disease-modifying therapies, β-cell preservation, cellular replacement, precision medicine, and artificial intelligence may further transform diabetes prevention and management.