From the chemical point of view, Vitamin D is a fat-soluble compound that resembles steroid hormones. Indeed, it is classified as secosteroid hormone and regulates several metabolic pathways: on one hand, it concurs to maintain calcium, magnesium and phosphate homeostasis; on the other hand, it has immunomodulatory, anti-proliferative and anti-inflammatory effects in different microenvironments of the body. Despite the accumulating attention about Vitamin D supplementation in human reproduction, data published so far still remain elusive. Nevertheless, available evidence suggests that Vitamin D excess plays a detrimental role both on female and male fertility: in women, it disturbs the physiological process of oocyte maturation and embryo quality; in men, it decreases spermatozoa count, their progressive movement and increases morphology abnormalities. These data urge us to recommend avoiding “empiric” supplementation of Vitamin D without a clear indication. In addition, our critical appraisal allow us to highlight that Vitamin D should be supplemented only during the luteal phase of the menstrual cycle; considering that this Vitamin has a clear progesterone-like activity, it may play a beneficial role on endometrial receptivity and support embryo implantation during early pregnancy. On the other side, its supplementation during the follicular phase does not have any specific indication and has a negative impact on fertility outcomes.