This chapter provides a brief account of the development of studies on structure, composition, properties, cytopathology, and taxonomy of Closteroviruses. The host ranges of individual Closteroviruses are typically rather narrow and the diseases produced are generally of the yellows type due to phloem necrosis and severe derangement of the chloroplasts. Beet yellows virus (BYV), like many Closteroviruses, is semipersistently transmitted by aphids. Recently, a mealybug has been confirmed as the vector of one Closterovirus and whiteflics have been reported as the vectors of several Clostero-like viruses. The RNA of BYV is infectious and that of citrus tristeza virus has been shown to act directly as a messenger in a rabbit reticulocyte translation system. Thus presumably these viruses and probably all Closteroviruses have positive-sense RNA. Closteroviruses, in common with many kinds of viruses, induce the range of pathological changes that is generally found in chlorotic and necrotic tissues.