The objective of gene therapy for the treatment of cancer is to kill tumour cells but preserve normal tissue; therefore, the ideal gene therapy agent would be targeted for specific transduction of tumour cells and have specificity in its cytotoxic action. A variety of strategies to achieve these aims have demonstrated promising results in the laboratory, including enzyme-pro-drug activating systems, correction of genetic mutations contributing to the malignant phenotype and stimulation of a T-cell-mediated anti-tumour immune response. The key to the success of all these strategies is an effective vector that can direct appropriate expression of the therapeutic gene. Viruses have many properties that can be adapted to achieve this therapeutic endpoint; furthermore, they can be engineered to replicate selectively in cancer cells and lyse them. The challenge now is to translate these features into effective therapies that can supplement or supplant existing treatments.