The two main species of human hookworm, Ancylostoma duodenale and Necator americanus (Table 1), are together believed to infect about 900 million people - mainly in tropical countries where adequate sanitary facilities may be lacking. But interactions between the two species, and their relative contributions to observed age-related infection patterns and seasonal cycles of transmission, continue to engender controversy. People tend to remain susceptible to infection through-out life, even with constant exposure to the infective stages. So what role does human immunity or resistance play in the epidemiology and control of infection? In this article, Jerzy Behnke reviews the epidemiology of hookworm infection in the light of current understanding of mechanisms involved in host responses to infection and hookworm evasion of those responses. As he stresses, much further work is required.