In recent years, among the many factors that cause cancer, tumor-bearing viruses have attracted particular attention from scientists. Among them are viruses of amphibians, reptiles, birds, mammals (including humans). How these viruses were discovered, how they act on the cell, whether they really cause cancer, what prospects does the study of tumor viruses open up to find out the essence of the cancer process and fight it - a book designed for a wide range of readers interested in the achievements of science is devoted to all these questions.