To understand the importance of vitamin D for our health, a historical perspective is useful. Striking parallels can be found in the research of the bone disease rickets and, centuries later, cancer. For both types of diseases, hypotheses about the importance of sun and vitamin D were initially proposed based on careful clinical observations, which were first criticized and rejected by science, but later impressively confirmed. An example of this is an assumption often referred to as the “vitamin D-cancer hypothesis”, which was proposed by the American epidemiologists Cedric F. Garland, Frank C. Garland, and Edward D. Gorham in the early 80s of the last century. This postulates that the high mortality rates from colorectal, breast, and ovarian cancer observed in sun-poor regions of higher latitudes are due to the prevailing vitamin D deficiency as a result of low UV-B radiation. Current study results not only convincingly support this hypothesis. They also impressively demonstrate the great importance of vitamin D deficiency as a risk factor for the frequent occurrence and unfavorable clinical course of numerous other diseases, including infectious, cardiovascular, metabolic, and autoimmune diseases.