Parasitism is a lifestyle employed by over 50% of all known species at some point in their life histories. The book focuses on the functional biology of parasitism, primarily within an ecological context. The concept of parasitism is reviewed and analyzed, with special emphasis given to Crofton's quantitative approach. Discussions follow on the manner in which parasite population biology is influenced by both density-independent and density-dependent constraints. Consideration is then given to those systems in which parasites are known to affect host population dynamics; both epidemiological and epizootiological case histories are described. Life history strategies are described from the perspective of parasite reproduction and colonization, and host behaviour. The biology of parasitism is then emphasized at the infra-, component, and compound levels in vertebrate and molluscan hosts. Finally, host-parasite relationships are discussed from the standpoint of their genetics, micro- and macroevolution, and biogeography.