Vitamin D is extremely important for the proper functioning of the body. The most commonly known role of vitamin D is its participation in regulation of calcium-phosphate metabolism and bone mineralization. This role is crucial in the prevention of rickets in children and osteoporosis in the elderly. In recent years, numerous studies have confirmed the pleiotropic effects of vitamin D. Proper vitamin D levels in blood have a positive effect on overall health, thus reducing the risk of many diseases. Vitamin D plays, inter alia, a positive role in some diseases of the gastrointestinal tract (inflammatory bowel disease), nervous system (Parkinson disease, Alzheimer disease), and cardiovascular disease (atherosclerosis). Additionally, its positive protective effect in the case of neoplastic and immunological diseases has been noted. Some studies also confirm the relationship of vitamin D deficiency to obesity and depression. In the event of these diseases, it is possible to prevent disease and support the process of treatment by maintaining appropriate levels of 25(OH)D in the blood. Besides, sufficient blood vitamin D levels reduces the risk of developing respiratory tract infections and suppresses cytokine storm, which is responsible for most COVID-19 deaths. The aim of the study was to present the current state of knowledge regarding the role of vitamin D in the human body, especially in the context of the impact of its abnormal level on the development of various diseases.