Oncologic emergencies represent a wide variety of conditions that can occur at any time during the course of a malignancy, from an initial presenting manifestation in someone with an undiagnosed cancer, to end-stage incurable metastatic disease. Emergent conditions can also arise after a malignancy has been in remission for many years, even decades, so clinicians must be aware of any prior history of cancer in patients. Oncologic emergencies include conditions caused by the cancer itself or side effects of therapy. Emergent conditions include metabolic, cardiac, neurologic, or infectious disorders. Many of these emergencies are imminently life-threatening, and can occur in patients with curable disease (such as lymphomas or leukemias); however, many also present in patients with incurable advanced disease. Prompt recognition and treatment of these conditions can lead to markedly improved quality and quantity of life.