This chapter is on disorders of calcium and magnesium. Abnormalities of calcium and magnesium metabolism occur frequently in patients admitted to intensive care units (ICU). Contractions of muscles, impulse conduction within nerves, release of hormones in response to stimuli, coagulation of blood and activation of protein kinases and enzyme phosphorylation are functions of calcium. Hypocalcemia and Hypercalcemia are two disorders of calcium metabolism. Hypocalcemia is defined as a level of serum total calcium less than 8.5 mg/dL. Hypoparathyroidism, renal failure, intestinal malabsorption, nutritional deficiency of calcium and vitamin D dependent rickets are causes of hypocalcemia. Hypercalcemia is an increase in the serum level of calcium. Hormone or vitamin mediated, Malignancy and Drugs are the causes of Hypercalcemia. Magnesium is a cation found in abundance throughout the human body, second only to potassium. The magnesium in the body is distributed into bones (60%), muscles (20%), soft tissue (20%) and extracellular fluid (1%). Hypomagnesemia is the disorder of magnesium metabolism.