Despite their availability since 1975, monoclonal antibodies (MAbs) have failed to make a significant impact in the clinical treatment of patients outside of diagnostic assays for the measurement of tumor markers. Recently, however, studies have appeared demonstrating that MAb-directed imaging techniques may have a meaningful role to play in the diagnosis and treatment of patients with solid tumors such as colorectal cancer and malignant melanoma. Furthermore, second generation MAbs are appearing that mediate more specific binding characteristics, and experimental studies have suggested that genuine antitumor activity may not be far away. Finally, the recent development of newer molecules, such as chimeric and human antibodies, may lead to significant advances in MAb use. Specifically, allergic reactions to murine antibodies can be reduced or avoided altogether, and repeated doses can be administered safely. These newer MAbs appear to mediate cell lysis more efficiently and have been combined with chemotherapeutic agents and cell toxins. Greater understanding of host responses to MAbs and the development of anti-idiotype and anti-anti-idiotype antibodies have led to clinical trials that, it is hoped, will confirm the place of MAb therapy in clinical oncology.