Replication-competent viruses are used as selective cancer therapeutics and the mechanisms leading to tumor-specific replication and antitumoral efficacy are now becoming apparent. The specific viruses in development include tumor-targeting herpes simplex viruses, autonomous parvoviruses, Newcastle disease viruses and adenovirus. Information is also available on antiviral immunology and viral defenses against host-mediated immunity. This approach has many potential attributes, in addition to potential hurdles that must be overcome.