The major characteristics of seven families of oncolytic viruses are summarized. Recombinant strains of all the DNA viruses shown are currently in clinical trials, whereas among the RNA viruses shown, only recombinant MV is in clinical trials; recombinant poliovirus and VSV are in pre-clinical trials and non-engineered strains of reovirus and NDV are in Phase I–II clinical trials. HSV1, herpes simplex virus 1; MV, measles virus; NDV, Newcastle disease virus; VSV, vesicular stomatitis virus.