Human gene therapy involves the transfer of genetic material into cells of a patient, either in vitro or in situ, for the therapeutic purpose of correcting or ameliorating genetic defects, alleviating disease, inhibiting infectious agents or destroying cancer cells. After identification of an appropriate disease target (inherited or acquired) for treatment, a number of basic technical issues underlie any gene therapy strategy. These include the choice of genetic material to be transferred, target cell or organ for gene modification, delivery systems and representative animal models of the targeted human disease. Tumor gene therapy represents a somewhat special category for gene therapy because its intended goal is the destruction of tissue rather than its correction or preservation. For an experimental therapeutic, cancer represents a useful target due to the paucity of effective treatments and the terminal nature of the disease. Viral vectors, currently the most widely used agents for gene delivery, generally fall into two classes; defective, which are unable to replicate; and replication-competent, which are able to replicate and spread within defined cell types. The strategies employed for cancer gene therapy range from cytotoxicity, either direct or through a 'suicide' gene-producing, to induction of a host antitumor immune response. Gene therapy is rapidly moving into the clinic, yet its efficacy remains to be demonstrated. Research into the underlying fundamentals of gene therapy discussed in this review will be critical to the ultimate success of this therapeutic modality.