The regulation of plasma calcium levels is essential for the normal physiologic function of every cell. Parathyroid hormone (PTH) is the principal regulator of serum calcium and phosphate homeostasis. PTH is synthesized and secreted by the parathyroid chief cells in the parathyroid glands primarily in response to a decline in serum calcium levels. The causes of hypocalcemia can be broadly classified as inadequate PTH or vitamin D production, PTH resistance, or miscellaneous causes. The term "hypoparathyroidism" refers to a metabolic disorder in which hypocalcemia and hyperphosphatemia occur either from a failure of the parathyroid glands to secrete sufficient amounts of biologically active PTH, or from an inability of PTH to appropriately induce a biological response in its target tissues. The most common cause of acquired hypoparathyroidism is surgery, accounting for 75% of all cases. Nonsurgical causes of hypoparathyroidism include autoimmune, genetic variants, infiltrative, metastatic, radiation, mineral deposition, magnesium deficiency or excess or idiopathic. The objective of this chapter is to provide a comprehensive review of the physiology of calcium homeostasis, the causes of hypocalcemia, and the epidemiology of hypoparathyroidism. It is very important to determine the underlying cause of the hypoparathyroidism in order to effectively treat our patients and improve quality of life.