Vitamin D (VD) deficiency represents a global health problem affecting populations across all age groups, regardless of place of residence, sex, race, or body mass. Unlike other vitamins, it functions in the human body as a prohormone. Its active form, calcitriol, regulates calcium–phosphate homeostasis and exerts pleiotropic immunological, cardiometabolic, neuroprotective, and anticancer effects. The primary source of VD is cutaneous synthesis induced by UVB radiation, which under favorable conditions can fully meet the body’s requirements. Fatty fish constitute the main exogenous source of vitamin D, whereas other dietary components, supplementation, and fortified foods play only a supplementary role. VD status is influenced by numerous environmental factors, including latitude, season, altitude, cloud cover, air pollution, and surface albedo. Although latitude was formerly considered the main predictor of VD levels, other determinants are now recognized as equally important. Among biological factors, age, sex, BMI, body fat content, skin melanin level, and genetic profile play a significant role. Melanin reduces endogenous VD synthesis, and this process further declines with age. Obesity predisposes to deficiency, most likely due to sequestration of cholecalciferol in adipose tissue. Behavioral factors such as diet, supplementation, wearing body-covering clothing, use of sunscreens, and sun-exposure habits significantly affect serum 25(OH)D concentrations. Interactions between these factors further complicate the assessment of deficiency risk. In Scandinavian countries, deficiencies related to low sunlight exposure are effectively compensated by diets rich in fatty fish and mandatory food fortification. The most effective preventive strategy is cholecalciferol supplementation in individually adjusted doses. Modern lifestyle, characterized by limited sun exposure, is one of the main contributors to VD deficiency. Since measurement of VD metabolites is not performed as a screening test in the general population, identification of risk factors and appropriate adjustment of supplementation remain crucial in deficiency prevention.