An oncologic emergency is defined as any acute, potentially life-threatening event, either directly or indirectly related to a patient’s cancer (ca) or its treatment. It requires rapid intervention to avoid death or severe permanent damage. Most oncologic emergencies can be classified as metabolic, hematologic, structural, or side effects from chemotherapy agents. Tumor lysis syndrome is a metabolic emergency that presents as severe electrolyte abnormalities. The condition is treated with aggressive hydration, allopurinol or urate oxidase to lower uric acid levels. Hypercalcemia of malignancy is treated with aggressive rehydration, furosemide, and intravenous (IV) bisphosphonates. Syndrome of inappropriate antidiuretic hormone should be suspected if a patient with ca presents with normovolemic hyponatremia. This metabolic condition usually is treated with fluid restriction and furosemide. Febrile neutropenia is a hematologic emergency that usually requires inpatient therapy with broad-spectrum antibiotics, although outpatient therapy may be appropriate for low-risk patients. Hyperviscosity syndrome usually is associated with Waldenström’s macroglobulinemia, which is treated with plasmapheresis and chemotherapy. Structural oncologic emergencies are caused by direct compression of surrounding structures or by metastatic disease. Superior vena cava syndrome is the most common structural oncological emergency. Treatment options include chemotherapy, radiation, and IV stenting. Epidural spinal cord compression can be treated with dexamethasone, radiation, or surgery. Malignant pericardial effusion, which often is undiagnosed in capatients, can be treated with pericardiocentesis or a pericardial window procedure.