Hypokalemia is a potentially life-threatening complication that occurs in many patients with cancer. Nurses need to be aware of the many common causes of this electrolyte disorder as well as appropriate assessment strategies that will enable potential problems to be detected and treated early. This article begins with a review of normal potassium balance and physiology, followed by a discussion of potential causes of hypokalemia in patients with cancer. These causes are grouped into four categories: inadequate dietary intake, extrarenal losses, redistribution abnormalities, and renal losses. Common causes of hypokalemia in patients with cancer are illustrated with case examples. Signs and symptoms associated with hypokalemia are discussed. Finally, implications for nursing assessment and intervention related to the administration of potassium are reviewed.