What influence does vitamin D have on the development of cancer, cardiovascular diseases, metabolic disorders, and autoimmune diseases? How can one achieve a balance between necessary sun exposure and the avoidance of skin cancer risk? This work provides answers to these and many other questions about this important vitamin, as well as the vitamin D deficiency disease rickets. It dispels common misconceptions about the structure, function, and effects of vitamin D and additionally provides numerous practical facts for optimal supplementation. The author is a recognized expert in the field and has been engaged in vitamin D research for over three decades. The book is aimed at physicians who wish to optimally care for and inform their patients based on the latest scientific findings regarding vitamin D. The Author Professor Dr. Jörg Reichrath is the senior consultant at the Department of Dermatology and the head of the Center for Clinical and Experimental Photodermatology at the Saarland University Hospital. The translation was done with the help of artificial intelligence. A subsequent human revision was done primarily in terms of content. This book is a translation of an original German edition. The translation was done with the help of artificial intelligence (machine translation by the service DeepL.com). A subsequent human revision was done primarily in terms of content, so that the book will read stylistically differently from a conventional translation.