Diabetes mellitus includes a group of metabolic disorders characterized by chronic hyperglycemia that occurs as a result of impaired insulin secretion and impaired insulin action. Type 1 diabetes mellitus is the most common form of diabetes in children and adolescents, and the frequency is increasing year by year. The appearance of type 2 diabetes mellitus in children and adolescents is closely related to the global obesity pandemic. Type 1 diabetes mellitus is characterized by an absolute lack of insulin, which in children and adolescents is most often the result of autoimmune destruction of the β-cells of the pancreatic islets of Langerhans. It is most often treated with insulin. Type 2 diabetes mellitus is caused by numerous factors, and is characterized by hyperglycemia as a result of reduced insulin secretion with previous insulin resistance. It is most often treated with metformin. Obesity is a significant risk factor for the development. It is important to prevent and treat it in order not to increase the morbidity of the individual. The most common mechanism of its occurrence is the imbalance between energy intake through food and energy consumption in the body. Puberty plays a role in the development of type 2 diabetes mellitus in children primarily due to the worsening of insulin resistance, which is caused by changes in hormonal homeostasis, primarily by hypersecretion of growth hormones. Caring for people with diabetes is a multidisciplinary team effort in which the most important thing is the patient's active involvement in treatment and self-control. The basis of any diabetes treatment is a healthy diet, exercise and a healthy lifestyle, which requires continuous education of the patient. Psychosocial problems are frequent in children with diabetes and their families. Prevention is not yet well known, but it is possible. Treatment of obesity and diabetes mellitus should become the main goals of public global intervention programs.