The experimental results from the use of gene transfer techniques for cancer therapy are promising but, at the same time, somewhat perplexing. Progress will depend on the further understanding of the biology of tumour interaction with the immune system, as this would be important for the development of optimal strategies for harnessing an effective immune response against tumours. On the other hand, extrapolation from laboratory models to human disease is limited and, therefore, there is a requirement for well-planned clinical studies. Although numerous viral and non-viral methods of gene transfer are available currently, in order to achieve realistic therapeutic efficacy for in vivo gene therapy of cancer there is a pressing need to improve on existing vectors. Hopefully, effort being directed at this objective will result in the development of safer, more highly efficient and well-targeted vectors which in turn will lead to the reality of wide clinical application.