Tumor lysis syndrome (TLS) is an oncology emergency that occurs as a result of rapid tumor cell breakdown and the consequent release of massive amounts of intracellular contents, including potassium, phosphate, and uric acid, into the systemic circulation. These metabolic disturbances lead to life-threatening conditions and may cause sudden death if not treated. TLS commonly occurs following initiation of cytotoxic treatment in patients with high-grade lymphomas or acute lymphoblastic leukemia. Spontaneous cases involving both solid and hematologic tumors have also been reported. Rarely, TLS occurs following treatment with irradiation, corticosteroids, hormonal therapy, or biologic therapy. It is necessary to identify patients at risk for TLS early in order to initiate preventive measures. In the event that preventive measures fail, the clinical parameters and signs of TLS must be understood and recognized so that treatment can begin as soon as possible, as this condition is a significant cause of morbidity and mortality.