Many indications are discussed for vitamin D substitution, such as osteoporosis, autoimmune diseases, cancer and psychiatric diseases. Also discussed is the fact that the majority of the German population suffer from a vitamin D deficiency. Review of the study results for these individual diseases and a critical analysis of the currently established therapeutic reference range, which defines a vitamin D deficiency. A literature search was carried out in the statements of the German Society for Nutrition, in scientific publications and journals. The study results on prevention and therapy of various diseases with vitamin D show inconsistent results. Well-established indications are the prevention of rickets in babies and the supportive therapy for osteoporosis. The currently established reference range for the definition of a vitamin D deficiency came from studies where vitamin D deficiency was correlated to an increase in parathyroid hormone. Different laboratories use different methods for measurement of vitamin D levels. More studies are needed to clarify the role of vitamin D in the prevention and treatment of various diseases. Another problem is that different laboratories do not use the same measurement methods to determine vitamin D and the use of different methods leads to widely varying results which cannot be compared. Therefore, a standardization of the methods would be desirable.