Antibodies have for many decades been viewed as ideal molecules for cancer therapy. Although promising from the start, it has taken much of more than two decades to reach the level of clinical application. Genetic engineering of antibodies; that is novel technologies for chimeric or humanizing monoclonal antibodies, has greatly advanced their utility in molecular targeting therapies, and in the past four years some therapeutic monoclonal antibodies for hematologic malignancies and solid tumors, such as Rituximab for B-cell lymphoma and Trastuzumab for metastatic breast cancer, have provided sufficient efficacy and safety to support regulatory approval from the U.S. Food and Drug Administration. They were subsequently approved by the Japanese Ministry of Health, Labour and Welfare in 2001. Many molecular biological and immunological studies have revealed the targeting properties of the host immune system and the biological mechanism of cancer cells for a more specific anticancer effect. Many clinical trials of monoclonal antibodies as a single agent, or in combination protocol with current standard chemotherapy or immunoconjugates have shown promise in the treatment of specific diseases. Furthermore, novel antibody designs and improved understanding of the mode of action of current antibodies lend great hope to the future of this therapeutic approach. The accumulating results from many basic, clinical and translational studies may lead to more individualized therapeutic strategies using these agent directed at specific genetic and immunologic targets.