Many readers may have only a vague idea about vitamin D. This is made complicated, in part, because it is also expressed with suffixes such as vitamin D(2) or vitamin D(3). Otherwise the prefix of "active" is also occasionally used. Vitamin D is often referred to as an important nutrient for calcium intake, especially for growing children and the elderly. On the other hand, it serves as a therapeutic drug for osteoporosis and psoriasis. Recent studies have suggested an association with a number of diseases such as cancer, diabetes and Alzheimer's disease. The author has been involved in the research and development of vitamin D applications for over 25 years, and has often witnessed even world-class experts confuse the two roles - vitamin and hormone - of "substance" D. Assuming some readers are not familiar with vitamin D, D hormone or osteoporosis, an outline of vitamin D and D hormone is delineated with a particular focus on the treatment of osteoporosis. Furthermore, development of alfacalcidol as the first prodrug of D hormone (calcitriol) and eldecalcitol as a characteristic new D hormone derivative and basic relationship between calcemic activity and effect on bone in vitamin D (cholecalciferol), D hormone (calcitriol/alfacalcidol), and a new D hormone derivative (eldecalcitol) are introduced.