This chapter examines the relationship between atopic disorders and parasitic infections. Atopy is an exaggerated IgE-mediated Type-1 immune response in predisposed individuals. Conflicting information exists in regard to the relationship of parasitic infections and the classic allergic diseases, that is, atopic dermatitis, allergic rhinitis and asthma. Attention is paid to the explanations for these discrepancies in the literature found within both human and animal studies on atopy with particular emphasis on helminthic infections. The factors that cause only a proportion of atopic individuals to develop clinical disease have not been defined although helminths confer protection in many studies examined. Early childhood infections help induce a Th1-biased immunity and prevent the induction of the Th2 system that causes atopy. Acute parasitic infections may increase manifestations of allergy, whereas chronic infections with parasites decrease atopic predisposition. Nonetheless, a causal association between geohelminth infection and atopic disorders has not been established. Some helminthic substances, especially the cytokines, have respiratory and anti-allergic effects, and may therefore become useful as therapeutic modalities for many atopic and allergic disorders.