This book addresses a very interesting topic and offers a novel insight of the well-known Vitamin D. In the past, this molecule has been wrongly considered from health professionals and researchers a “Vitamin”. The scientific community has now extensively accepted that calcitriol, commonly called “Vitamin” D, is a steroid (more precisely, a secosteroid) hormone. In this book series, the third volume titled “Vitamin D as Progesterone-like Hormone - Novel Applications in Obstetrics and Gynecology”, offers an innovative overview of the critical role played by Vitamin D in the therapeutic approach to common obstetric and gynecological disorders. The nuclear receptors of steroid hormones have common structure/function domains called “nuclear receptors superfamily”. Progesterone and Vitamin D receptors belong to two different classes of this superfamily. Throughout each chapter, we need to keep in mind this relationship as the core of our book. Vitamin D exerts various and distinct physiological activities during gestation along with progesterone; in some cases, it may act also as its substitute or they may strengthen reciprocally. Like progesterone, Vitamin D seems extremely functional during the luteal phase onwards. A lot of research needs to be done in order to further investigate this novel fascinating role of Vitamin D. We can see the multiple activities also in combination with other molecules, such as alpha-lipoic acid and magnesium for controlling the uterine contractile activity, inositol and melatonin to support the luteal phase or epigallocatechin gallate in the management of uterine myomas. All these attractive insights will be discussed in the present book, analyzing in depth each aspect of this powerful molecule, bringing the instructions from this guide to everyday practice.