This chapter provides a brief account of the development of studies on structure, composition, properties, cytopathology, and taxonomy of some unclassified viruses. Oat blue dwarf virus (OBDV) has small polyhedral particles about 30 nm in diameter which sediment as a single component of 119S. In 1977, R. Hull suggested that phleum mottle virus (PhMV) and a number of other isolates formed a cluster of viruses with similar properties which were good candidates for a new taxonomic group. Panicum mosaic virus (PanMV) is serologically related to PhMV, but the relationship is so remote that the two viruses can be safely considered as distinct. Hull pointed out the similarities between carnation mottle virus, pelargonium flower break virus, elderberry latent virus, and narcissus tip necrosis virus and suggested that this cluster of viruses was a suitable candidate for a new taxonomic group.