Tumor immunotherapy depends on the interactions between the host, the tumor, and the immune system. Recent data suggests that priming of antigen-specific T cells alone may not be adequate for mediating regression of established tumors because of the immune inhibitory influences within the tumor microenvironment. Thus, we developed a recombinant vaccinia virus vector to express single or multiple T cell costimulatory molecules as a vector for local gene therapy in patients with malignant melanoma. This approach is feasible and generated local and systemic tumor immunity and induced objective clinical responses in patients with metastatic disease. This chapter reviews the details and major issues related to using live, replicating, recombinant poxviruses for gene delivery and antitumor vaccination within the tumor microenvironment.