This chapter discusses the diagnosis, evaluation and management of common electrolyte disorders that include hyponatremia, hypernatremia, hypokalemia, hyperkalemia, hypocalcemia, hypercalcemia, hypomagnesemia and hypermagnesemia. The signs and symptoms of moderate hyponatremia are non-specific such as generalized weakness, lethargy, nausea, vomiting, and muscle cramps. Hyponatremia is most commonly caused by an excess of antidiuretic hormone (ADH) released in response to intravascular volume depletion, exacerbated by volume replacement with hypotonic fluids. Overly rapid correction of hypernatremia may lead to cerebral edema and seizure. As with hyponatremia, to ensure a safe and accurate replacement rate, serum sodium levels have to be checked frequently. Release of calcium stores in the circulation is regulated by extracellular calcium concentration, parathyroid hormone (PTH), vitamin D metabolites, and calcitonin. Mild hypomagnesemia is usually asymptomatic, but failure to correct low serum magnesium may contribute to refractory hypokalemia and hypocalcemia.