Type 1 diabetes is an autoimmune disease that mostly affects children and is caused by the destruction of insulin-secreting pancreatic β-cells. The result is an increase in blood glucose levels, which leads to the development of a number of complications, and children lag behind in growth and development. Genetic predisposition plays an important role in the development of type 1 diabetes, but other factors such as the length of breastfeeding, early childhood infections, and others are also important. The incidence of type 1 diabetes is particularly high in Europe with growth of 3 % in the last 25 years, especially in the countries of the northern hemisphere. Vitamin D is a fat-soluble vitamin that has many important functions in the body and although traditionally associated primarily with bone health, it appears to have good potential in preventing type 1 diabetes. Its biggest source is the sun, but it can be ingested through foods like fatty fish or milk as well as cereals. Serum concentrations of 25-hydroxyvitamin D (25-OHD) are highly dependent on environmental factors, from dietary vitamin D intake (<20 % in the form of ergocalciferol D2 and colecalciferol D3) to sunlight exposure that stimulates colecalciferol synthesis in the skin. Due to the lack of vitamin D, the most common disease that occurs in children is rickets, but there are other diseases such as osteomalacia, multiple sclerosis, inflammatory bowel disease or rheumatoid arthritis. Vitamin D is also important in the treatment of type 1 diabetes. Numerous studies have shown that there is a link between vitamin D deficiency and an increased risk of type 1 diabetes, and supplementary vitamin D is being recommended, especially for pregnant women and infants. Also, many countries have introduced food fortification with vitamin D, mainly milk and dairy, cereals and juices.