Tumor lysis syndrome (TLS) is a potentially fatal complication of induction therapy for several types of malignancies. Electrolyte derangements and even downstream complications may also occur prior to the initial presentation to a medical provider, before an oncologic diagnosis has been established. It is therefore imperative that emergency physicians be familiar with the risk factors for TLS in children as well as the criteria for diagnosis and the strategies for prevention and management. Careful evaluation of serum electrolytes, uric acid, and renal function must occur. Patients at risk for TLS and those who already exhibit laboratory or clinical evidence of TLS require close monitoring, aggressive hydration, and appropriate medical treatment.