The immune system provides an attractive option for use in cancer therapy. Our increasing understanding of the molecular events important in the generation of an effective immune response presents us with the opportunity to manipulate key genes to boost the immune response against cancer. Genetic modification is being employed to enhance a range of immune processes including antigen presentation, activation of specific T cells, and localization of immune effectors to tumors. In this review, we describe how many diverse cell types, including dendritic cells, T cells, and tumor cells, are being modified with a variety of genes, including those encoding antigens, cytokines, and chemokines, in order to enhance tumor immunity.