This book discusses the bidirectional relationship between vitamin D and mineral ions, detailing how these interactions form a physiological feed-forward loop essential for maintaining health, which becomes dysregulated in various disease states. This book compiles the roles and regulatory mechanisms of vitamin D and mineral ions in human health and disease. The chapters focus on the diverse physiological functions of vitamin D and the critical role of magnesium in maternal–fetal development and the importance of calcium in the gut–glucose axis. The overall content of the book reemphasizes the essential roles of vitamin D and mineral ions in maintaining normal physiological functions across multiple organ systems. Vitamin D is requisite for physiological functions of cells and organs. Vitamin D facilitates the absorption of essential minerals, including calcium, phosphorus, magnesium, copper, zinc, iron, and selenium. While certain minerals, including magnesium and zinc act as cofactors to activate vitamin D. Similarly, calcium and phosphorus are involved in vitamin D-mediated signaling events. Vitamin D also enables the uptake of toxic elements, including lead, arsenic, aluminum, cobalt, and strontium. Although, vitamin D and calcium interactions are well described in the literature, more generalized roles, and regulation of vitamin D on mineral ion metabolism is not well-defined. This book will serve as a concise reference on vitamin D regulation and its diverse biological functions, including roles in mineral ion homeostasis, antimicrobial activity, immune modulation, and sleep regulation. It also addresses the clinical implications of vitamin D in brain health, fracture healing, and tumorigenesis. The content is accessible to readers with minimal prior knowledge of the field.