Barley Yellow Dwarf (BYD) is a serious Luteovirus disease that affects small grain production worldwide. The aphid-transmitted virus (BYDV) infects practically all members of the Graminae (Poaceae) and is responsible for serious losses in cultivated species such as barley, wheat and oats. The study of BYD is complex, as it involves interactions among a vector, a plant and a virus. Hence, symptom expression is highly dependent on environmental conditions, serotypes plant genetic background and physiological stage of inoculation. Consequently, tolerance to BYDV is also difficult to study and understand. This review explores the basic biology of BYD, its symptoms, its viruses and yield losses it can cause.