Some information about helminths, the diseases they induce, and approaches to their control have general application to each of the nine groups discussed in this handbook (Chapter 5 to Chapter 13). Each species of helminth considered here to be an agent of disease lives for part of its development or reproductive life inside the human body. The dynamic relationship between helminths and their human hosts exemplifies parasitism. Each is a partner of a pair of interacting species that, through evolutionary progression, have acquired integrated genomes so that the parasitic species is dependent on at least one gene or gene product of the host species (MacInnes, 1974). And at some stage in its life history, a parasite is more than likely to obtain nutrients and energy from the food, metabolites, or tissues of its host.