Virotherapy: The viruses that can cure. The viruses that can selectively replicate in and destroy neoplastic cells are called oncolytic. The idea of using these viruses as anticancer drugs goes back to the beginning of the 20th century. Since than various oncolytic viruses have been taken to clinical trials. Oncolytic viruses can be classified largely into two groups: naturally oncolytic viruses (mainly RNA viruses) and genetically engineered viruses to achieve councer specificity (usually DNA viruses). The review is focused on the basic scientific principles used for development of oncolytic viruses and summarize current results from clinical trials. The eligibility, feasibility and safety of oncolytic virus therapy as a novel therapeutic agent against cancer is discussed.