Hypocalcemia may be presently defined as a serum calcium value below the reference range, usually below 8,4 mg/dL, in neonates as well as adults. This alteration may remain asymptomatic or be associated with severe clinical disorders. Its causes include classical pathologies such as vitamin D deficiency, hereditary or acquired resistances to vitamin D, hypomagnesemia, hypoparathyroidisms and resistances to parathyroid hormone, renal insufficiency and massive cellular lysis. They also include iatrogenic causes as well as recently isolated diseases, like the hypocalcemic syndrome due to mutations in the calcium-sensing receptor gene. The management of hypocalcemia depends on its causes, clinical consequences, severity, and on the age of the patient.