A potassium disorder is the most common electrolyte abnormality in hospitalized patients. Hypokalemia and hyperkalemia are two main conditions of potassium disorder. The total body potassium content is approximately 50 mEq/kg and is distributed asymmetrically in the body. About 98% is intracellular and only 2%, or 65–70 mEq, is extracellular. Hypokalemia is defined as serum potassium concentration less than 3.5 mEq/L. Decreased potassium intake, transcellular shift of potassium from extracellular to intracellular compartments and increased potassium losses are causes of hypokalemia. Cardiac, skeletal muscle and gastrointestinal renal are organ system effects of hypokalemia. Hyperkalemia is defined as potassium level more than 5.5 mEq/L. It divides in to three hyperkalemia i.e. mild, moderate and severe. Reduced glomerular rate, effective hypoaldosteronism and transcellular shift are three main mechanism of hyperkalemia. Pseudohyperkalemia and decreased potassium elimination are causes of hyperkalemia. Cardiac effects, neuromuscular effects and metabolic effects are clinical manifestations of hyperkalemia. The treatment of hypokalemia and hyperkalemia are given in this chapter.