With up to 56% of individuals taking diuretics likely to develop hypokalemia, and comorbid disease and many other types of medications having the potential to induce hyperkalemia, potassium abnormalities are some of the most commonly seen electrolyte abnormalities in the emergency department (ED). Unless recognized and treated appropriately, they can also be some of the most deadly. Symptoms accompanying potassium abnormalities are often vague, involving multiple organ systems. This evidence-based review discusses the etiology, differential diagnosis, and diagnostic studies for detecting hypokalemia and hyperkalemia, including managing laboratory errors that lead to factitious potassium findings. Recognition and treatment of life-threatening dysrhythmias in hypokalemia and hyperkalemia are key to managing these potassium abnormalities. Electrocardiogram (ECG) findings, treatment algorithms, and controversies on treating potassium abnormalities in the ED are discussed, with recommendations on criteria for disposition.