Hypokalemia and hyperkalemia are the most commonly encountered electrolyte abnormalities in hospitalized patients. Because untreated hypokalemia or hyperkalemia is associated with high morbidity and mortality, it is important to recognize and treat them immediately. Hypokalemia and hyperkalemia can result from disruptions in transcellular homeostasis or in the renal regulation of K⁺ excretion. Although the recognition is simple, appropriate management requires an understanding of normal K⁺ homeostasis and pathophysiology. In this article, normal K⁺ homeostasis, pathophysiology, diagnosis and management of hypokalemia and hypokalemia are discussed.