Differences in antigen expression between tumor cells and normal cells can be exploited to develop a variety of therapeutic anticancer agents. For example, vaccines containing tumor antigens can be administered to patients to elicit an immune-mediated attack against the tumor. Tumor antigens can also be targeted by passive transfer of monoclonal antibodies (MoAbs), or MoAbs that have been modified to carry toxin molecules. A large number of these antigen-specific agents are currently in clinical or late preclinical development. For the anticancer vaccines, much of the current development is focused on determining optimum immunization approaches. In the case of antibody-based reagents, second- and third-generation constructs are being developed to improve potency, decrease immunogenicity, and increase distribution to tumor.