Monoclonal antibodies used in the treatment of cancer are discussed. Monoclonal antibodies are a new class of agents targeted at specific receptors on cancer cells. In addition to having direct cellular effects, antibodies can carry substances, such as radioactive isotopes, toxins, and antineoplastic agents, to the targeted cells. Five monoclonal antibodies--rituximab, trastuzumab, gemtuzumab ozogamicin, alemtuzumab, and ibritumomab tiuxetan--are available for clinical use. Rituximab is active against indolent lymphomas, providing a valuable alternative for patients with relapsed or refractory disease. Rituximab plus cyclophosphamide-doxorubicin-vincristine-prednisone (CHOP) increased survival over CHOP alone in patients with high-grade lymphomas. Trastuzumab has significant activity against HER-2-positive breast cancer, especially in combination with paclitaxel or an anthracycline and cyclophosphamide. Gemtuzumab ozogamicin is an active second-line therapy in older patients with acute myelogenous leukemia, but its role in combination regimens is unclear. Alemtuzumab is a valuable option for salvage therapy of patients with chronic lymphocytic leukemia. Ibritumomab tiuxetan delivers radioactive isotopes to tumor cells and is active against indolent lymphomas in patients who have relapsed after chemotherapy or rituximab therapy. The most common adverse effects of monoclonal antibodies are myelosuppression, infusion-related reactions, and hypersensitivity reactions. Rituximab may cause tumor lysis syndrome, arrhythmias, and pulmonary dysfunction. Alemtuzumab causes immunosuppression, increasing the risk of infection. Gemtuzumab ozogamicin may cause hepatotoxicity, and trastuzumab may cause significant pulmonary or cardiac toxicity. Investigational monoclonal antibodies include edrecolomab and tositumomab. Monoclonal antibodies have a significant role in the management of patients with advanced refractory or relapsed lymphomas and leukemias.