Tumor-associated antigens (TAAs) have been formulated into vaccines that, combined with adjuvants, cytokines or other strategies to boost the immune response, are now in clinical development. Both humoral and cellular immune responses to TAAs have been generated using these vaccines. This approach relies on the patient's own immune system generating an effective anti-tumor immune response. The advantage of this over therapy with monoclonal antibodies for the treatment of cancer is that multiple antigenic epitopes can be involved and the immune system is able to adapt to the most effective antigenic specificity for tumor growth control and rejection. In this article, we describe the clinical use of vaccinia virus, in particular modified vaccinia virus Ankara (MVA), to express TAAs in vivo and to stimulate an effective immune response to the cancer antigens.