Respiratory disease is commonly encountered in sheep flocks, affecting groups or individuals. It often involves a combination of infectious causes as well as predisposing management factors, potentially leading to significant losses. However, cases of respiratory disease represent only 5.6 per cent of sheep submissions received by the Veterinary Laboratories Agency – a figure that is likely to be lower than the actual prevalence. In many instances, a presumptive diagnosis will be made following clinical and on-farm postmortem examinations. This article discusses the causes of respiratory disease in sheep and describes the diagnostic procedures that can be performed, with a checklist of potential pathogens for veterinary surgeons in the field. An article in the next issue will describe methods for its control and treatment.