During the past century, many attempts have been made to exploit the ability of some viruses to infect and destroy cancer cells. Crippled, non-replicative viruses have been used as vectors to transfer genes into tumours. Both strategies have serious limitations. The time is now ripe, however, for full convergence of these two research tracks. On the one hand, the intratumoral propagation of replicative viruses would overcome the low levels of gene transfer achieved by current viral vectors. On the other hand, the versatility provided by vectors encoding foreign genes, which are limited in their uses only by our ingenuity, would overcome the physiological barriers to robust propagation of the viral progeny in the tumour. This empowering synthesis will provide truly new opportunities that might realise the promises of gene transfer for the therapy of cancer.