Oncologic emergencies can threaten the lives of children with cancer even before a diagnosis is made. Awareness of common or life-threatening toxicities of cancer and cancer treatment facilitates rapid diagnosis and appropriate immediate management. Clinicians in both primary and tertiary care must recognize and manage five types of oncologic emergency: masses affecting vital organs (superior vena cava [SVC] syndrome, central airway compression [CAC] syndrome, cardiac tamponade, pleural effusion, spinal cord compression, and intracranial hypertension), metabolic emergencies (tumor lysis syndrome [TLS], hypercalcemia, hyponatremia, lactic acidosis, adrenal insufficiency), gastrointestinal emergencies (typhlitis, intestinal obstruction, bowel perforation), and chemotherapy extravasation. For new patients with suspected cancer, a careful history to elicit signs and symptoms, a physical examination, chemistry profile, complete blood count, and chest radiograph are sufficient to rule out an oncologic emergency. Any neurologic deficit, respiratory distress, altered vital signs, abdominal tenderness, abnormal laboratory values, or mediastinal mass on chest radiograph should prompt a thorough and immediate evaluation for life- and organ-threatening emergencies, which are the subject of this chapter.