Viral vectors are under development for anticancer therapy. As they can infect tumours and activate the immune system, viral vectors may directly destroy cancers (oncolysis), deliver genes with antitumour activity directly to the cancer cells, or act as cancer vaccines. Better insights into the biology of the various vectors in use (e.g., poxvectors, adenovirus, adeno-associated virus, reovirus, Newcastle disease virus) are making it possible to engineer viruses that are more tumour-specific, efficient at tumour infection, and which have enhanced safety due to incorporation of safeguards should dissemination occur. As considerable research has focused on therapy of colon cancer with viral vectors, this review will illustrate the major concepts of viral therapy of cancers with examples from studies targeting colorectal carcinoma.