Barley yellow dwarf (BYD) is a widespread and economically important disease of cereal crops caused by a group of closely related viruses in the Luteoviridae family. Viruses are transmitted by various aphid species that feed on the phloem of infected plants. This disease affects the growth, development, yield, and quality of barley, wheat, oats, maize, rice, and other grasses. The symptoms of BYD include stunting, leaf discoloration, reduced root growth, delayed or absent heading, and an increased susceptibility to secondary infections. Disease severity depends on several factors, such as the virus strain, host cultivar, time of infection, and environmental conditions. This chapter provides a comprehensive overview of the taxonomy, geographic distribution, symptoms, pathogenicity, epidemiology, and isolation and control measures for BYD. We also discuss the current challenges and future prospects for managing this disease in a changing climate and agricultural landscape.