Prokaryote viruses include 14 officially accepted families and at least five other potential families awaiting classification. Approximately 5,500 prokaryote viruses have been examined in the electron microscope. Classification has a predictive value and is invaluable to control experimental techniques and results. In describing viruses, the choice of methods depends on structure and taxonomical position of viruses. The study of isometric, filamentous, and pleomorphic viruses requires more detailed investigations than that of tailed species.