In this review the term “ectoparasite” is used in its strict sense as being an arthropod that is closely associated with its vertebrate host for all or extended parts of its life cycle. Certain other blood-feeding insects are referred to where they are considered of particular interest. The scope includes brief life histories of the major groups of ectoparasites, the distribution of the species on its individual host, the feeding mechanisms of the various groups, effects of the nutrition and endocrine state of the host on its ectoparasites, and a discussion of toxins produced or secreted by these parasites during feeding. Another review on host-ectoparasite relations is planned, the scope of which will include pathogenesis, immunologic phenomena, native and acquired resistance, and genetic considerations.