This chapter provides a brief account of the development of studies on structure, composition, properties, cytopathology, and taxonomy of pea enation mosaic virus (PEMV). PEMV is transmitted by aphids in a persistent manner, but the virus probably does not multiply in the vector. The virus can also be experimentally transmitted by mechanical inoculation. Cells of PEMV-infected plants contain virus particles scattered in the nuclei and to a lesser extent in the cytoplasm and vacuoles; sometimes the particles also form crystalline arrays in the cytoplasm. The proposal that PEMV replicates in the nucleus also finds support from the results of biochemical studies. It has been shown that both PEMV-specific double-stranded RNA and virus-induced RNA-dependent RNA polymerase are located in nuclei of infected cells. PEMV, which is circulative in its aphid vector, has been studied extensively in thin sections of infective insects.