Calf diarrhoea is an important cause of economic loss. The aetiology of the disease is considered with particular reference to the role of viruses. Although many microbial organisms have been associated with the disease, there is doubt concerning their true role as causative agents. Two viruses, neonatal calf diarrhoea reovirus-life agent and calf coronavirus, have been discovered recently which appear to play an important role in many cases of diarrhoea and the evidence for considering them to be primary causes of the disease is discussed.