Gastrointestinal helminths are extremely numerous both in man and in animals where they cause severe impaired production. The deleterious effects of an infection is easily demonstrated with blood-sucking parasites: the daily loss of blood in sheep infected with Haemonchus contortus can exceed the equivalent of 100 ml (0.05 ml per worm per day) [11. These parasites have very different life cycles, but in each instance, their gastrointestinal developmental stage (often adult worms) has been clearly identified as one of the targets of immunity. Several important reviews on immunity in parasitic diseases have appeared in recent years [2-8] and the present one will attempt to classify the data in a rational, nonhistorical way.