Helminth parasites in small numbers infest a very large percentage of the birds of economic importance but the damage and loss for which they are responsible has never been calculated. In considering these parasites there are two schools of thought—the first considers every helminth as a possible pathological factor and cause of death. While our knowledge of avian diseases is still rather scanty this opinion is easily understood for birds often die showing lesions in the gut and the only obvious agent is the helminths present. Recently, however, opinion has swung in the opposite direction and given rise to a second school, in which there is a tendency to discount the dangers of helminthic infestation and to minimise their importance. In the Institute of Agricultural Parasitology some caution is always adopted at post-mortem examinations before diagnosing the cause of death as being definitely due to helminths for it is felt that there are many disease factors, imperfectly understood as yet, which probably have far reaching effects on their avian hosts. However, in certain cases pathological conditions have been noticed which were undoubtedly due to helminths.