Vitamin D is a group name for a group of vitamins which are soluble in fats. It is considered one of the oldest prohormones on earth. The most common types of vitamin D are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Sources of vitamin D2 are of plant origin, whereas vitamin D3 sources are of animal origin. The active form of vitamin D is calcitriol (1,25-dihydroxycholecalciferol, 1,25(OH)2D). The main source of vitamin D is the sun, more precisely its UVB radiation. It needs to be clear that sunbathing in the means to synthesize vitamin D needs to be done without sunscreen, but avoiding sunburn. In optimal conditions our skin can synthesize from 10 000 – 20 000 IU (international units) of vitamin D in 30 minutes, after that the synthesis of vitamin D naturally stops. In addition to playing a very important role in maintaining bone and immune system health, its benefits in many diseases are increasingly emphasized and it plays a very important role during pregnancy for both mother and fetus and also later for the newborn. A lot of people around the world have vitamin D hypovitaminosis which is diagnosed by measuring the concentration of 25 hydroxyvitamin D, 25(OH)D in the blood, more precisely in the serum. Due to the lack of vitamin D in the body, various complications and diseases can occur. On the other hand, overdoses of vitamin D in the body cause hypervitaminosis, so care should be taken not to exceed the recommended dose because otherwise it can cause unwanted side effects and toxicity. To avoid side effects, vitamin D should always be taken under medical supervision.