Avian reoviruses are ubiquitous among poultry flocks. Although infection is usually present without disease, reoviruses may occasionally be involved in several disease syndromes of whic h viral arthritis/tenosynovitis in chickens is the most important, particularly in broiler breeds. While reoviruses have been isolated from turkeys and several other species of birds with various conditions, the presence of the virus has been conclusively linked wit h disease in relatively f e w instances. In chickens in particular, avian reoviruses with a wid e spectrum of pathogenic capability have been isolated and several antigenic types exist. Diagnosis is dependent on the detection of the virus in clinical samples, although the presence of the virus does not necessarily confirm that this is the cause of the disease, except wher e reoviruses are detected in affected joints. Serological tests are usually difficult to interpret in view of widespread and frequently harmless reovirus infection.