Recent development of immunotherapy has led to remarkable advancement in cancer therapy. Drugs that inhibit the cytotoxic T-lymphocyte-associated protein (CTLA-4) and programmed death-1 (PD-1) immune checkpoint pathways have shown improved patient survival. However, by altering the immune response to fight cancer, a new class of adverse reactions has emerged, known as immune-related adverse events. These adverse events are due to overactivation of the immune system in almost any organ of the body, can occur at any point in a patient's treatment course, and may become life-threatening. This article describes how to promptly recognize and manage these toxicities.