It is a well-known fact that infection of a bacterium, plant or animal with one virus frequently prevents or partially inhibits simultaneous propagation of another viral agent in the same host. Such interference phenomena, which have been called also “sparing effect”, “cell blockade”, or “Schienenimmunität”, have aroused great interest on account of their theoretical, and possibly practical implications. As a result, the literature on this subject has been increasing rapidly in recent years. It may be profitable, therefore, to take stock at this time and to attempt formulation of general principles which have evolved in studies of the various phenomena. It will be apparent that the present knowledge is still very fragmentary, so much so that the plural in the title of this review is being employed advisedly because of the fact that it is not known whether the various instances of interference described are all based upon related mechanisms, or whether several entirely different reactions are involved in the various observations.